Changelings can both change themselves into an arbitrary imagined form and into forms of specific creatures they have seen, provided the form they are becoming have the same basic arrangement of limbs as their current form

The relevant text for the ability is:

As an action, you can change your appearance and your voice. You determine the specifics of the changes, including your coloration, hair length, and sex. You can also adjust your height and weight, but not so much that your size changes. You can make yourself appear as a member of another race, though none of your game statistics change. You can't duplicate the appearance of a creature you've never seen, and you must adopt a form that has the same basic arrangement of limbs that you have. Your clothing and equipment aren't changed by this trait [...]

Additionally, it's worth sign-posting that we will look at the description of the changeling race and the examples given of their abilities there for some of the later analysis we do.

There are a few important things to parse here:

1. The first line separates appearance and voice into two separate things that can be changed.
2. There are some restrictions on what you can change in terms of your appearance, however due to the explicit separation between appearance and voice in the first line, these appearance restrictions don't apply to voice changes.
3. You don't change your game statistics and must keep the same basic arrangement of limbs.
4. Taking the context of the description of the race into account helps us parse the specific restriction

   You can't duplicate the appearance of a creature you've never seen [...]

I'll go through each of these separately

Appearance and Voice are changeable separately and independently

The first line of the ability specifies

As an action, you can change your appearance and your voice.

This clearly and explicitly makes the changes to your appearance and to your voice two separate things. To put it another way, this sentence makes it clear that your voice, and your appearance, are two separate things for the purposes of this rule (by virtue of them being provided separately in a list).

We should interpret this to mean that you can change one, or the other, using this action. Even if you were to subscribe to the view that you need to change both at the same time, you can choose to "change" your appearance to be precisely the same as it currently is, while changing the voice that comes out of it, or vice versa, resulting in functionally the same outcome.

Your appearance changes have some restrictions, but these restrictions are only to your appearance (not your voice changes)

As mentioned, the first line makes appearance and voice two separate things for the purposes of this feature.

It then goes on to say

You can also adjust your height and weight, but not so much that your size changes. You can make yourself appear as a member of another race, though none of your game statistics change. You can't duplicate the appearance of a creature you've never seen, and you must adopt a form that has the same basic arrangement of limbs that you have.

Your voice changes are not mentioned anywhere in these restrictions and as a result, changes to your voice are not covered by these restrictions (by virtue of them being clearly identified as being separate from your appearance in the first line of the ability).

You can't add limbs you don't have, or gain new abilities using the change

There are two relevant restrictions here:

You can also adjust your height and weight, but not so much that your size changes. You can make yourself appear as a member of another race, though none of your game statistics change.

So, you can't change a size category, and you don't gain any new abilities that your assumed form would ordinarily have. You don't gain darkvision by making yourself appear to be an Elf and you don't gain the Kenku's Mimicry trait by making yourself appear to be a Kenku.

Duplicate means just that, duplicate, it does not mean you can't invent appearances that aren't based on a real person and change into those

One potential sticking point is this restriction on appearances:

You can't duplicate the appearance of a creature you've never seen [...]

Using the fact that D&D 5e is all rules text (or to put it another way, 5e doesn't have flavour text in the way 4e did) we need to examine the information we are provided around the Changeling race to enable us to interpret this restriction appropriately. Specifically of interest are these two passages:

Changelings

[...]

Changelings can shift their forms with a thought. Many changelings use this gift as a form of artistic and emotional expression. [...]

[...]

Masks and Personas

In their true form, changelings are pale, with colorless eyes and silver-white hair. It is rare to see a changeling in that form, for a typical changeling changes their shape the way others might change clothes. A casual shape - one created on the spur of the moment, with no depth or history - is called a mask. A mask can be used to express a mood or to serve a specific purpose and might never be used again. However, many changelings develop identities that have more depth. They build an identity over time, crafting a persona with a history and beliefs. [...]

Adding to this, since 5e uses natural language for it's rules, as opposed to strict legal text, we need to understand what duplicate actually means. Using the Cambridge, Merriam-Webster and Collins Dictionaries we have this meaning ascribed to the word (I'm quoting the Cambridge text, but all three ascribe this meaning):

• adjective: being an exact copy of something
• verb: to make an exact copy of something
• noun: something that is an exact copy of something else

Taking all of that together, we should interpret the statement:

You can't duplicate the appearance of a creature you've never seen [...]

To be "If you want to precisely copy your appearance to be that of another creature you have to have to have actually seen them. Looking at a painting of them, or receiving a description of them is not enough."

Importantly, if you are not trying to duplicate a creature, but just looking to change your features to something you have imagined, then you no restrictions are applied by this paragraph. Nicely, this interpretation is consistent with the idea from the race's description of changing their form being an expression of artistic intent or as a result of a strong emotional response. So no, you can't make yourself look like Laeral Silverhand without seeing her, however you can make yourself look like a female human with long silver hair.

Conclusion

This ability gives you some pretty cool options:

1. You can change your voice independent of your appearance
2. When you are duplicating someone's voice, you can get the sound and specific speech defects or characteristics (like a lisp caused by a deformity, or their accent), however you can't by default duplicate (without specific study) the nuances of how they communicate. Communication involves spoken and non-spoken communication, and the voice only helps with some of the spoken part of the communication. For example:
   • You don't gain knowledge of phrases they would use regularly
   • You don't know things that might cause their speech to normally have a double meaning (like among groups of close friends the use of "swear" words can frequently be terms of endearment and familiarity as opposed to things designed to cause offense)
   • You don't know the way they would laugh (Think Jimmy Carr laughing on an in breath vs most people laughing on an out breath)
   • You don't know their mannerisms, like to they click their tongue after saying specific words, or they might nod their head or pull a face after saying specific things.
3. If you are trying to duplicate a specific creature's appearance you have to have seen them first. So you couldn't duplicate Drizzt Do'Urden unless you had seen them, either in person or through some form of magical sight (like scrying).
4. On the flip side, this doesn't restrict you from changing your appearance to make yourself look different, but you aren't trying to duplicate a creature. So you could choose to make your hair 2ft longer, or elongate your nose, or give yourself a cracked tooth or pointy ears, you could make yourself look like an elf, but not a specific Elf, or any combination of these or other cosmetic alterations. You can also match any voice you wish, with any appearance you wish, so you could have a slight female Baritone, or you could be an adult male Tiefling with the voice of a Kobold child.
5. You don't change your game statistics (as opposed to your voice and appearance)

Taking all of these together, our conclusion is that Changelings can match the sound and timber of someone's voice, but not their mannerisms. While they can make up their appearance to suit their will if they aren't trying to duplicate a specific creature (just working from their imagination), they can't gain new abilities from those changes. As a result, they can turn themselves into an imagined form, so long as that imagined form meets all of the restrictions that the ability outlines (is the same size, has the same basic arrangement of limbs, etc).

If they have never seen any Dwarves, then they are going to have a hard time using their imagination to come up with an accurate Dwarf form, but they can certainly attempt it.

To answer your specific sub-questions:

1. Eve can assume the form of Alice the human.
   • Yes
2. Eve can assume the form of an arbitrary human that does not necessarily resemble any actual human she has ever seen.
   • Yes
3. Eve can assume the form of a humanoid creature that resembles Bob the dwarf, although not necessarily correct in every detail.
   • Sort of. Eve can assume a form that resembles the description they have heard of Bob the Dwarf, and how that description has fed into her imagination of what Dwarves actually look like. They may get basic things about Dwarves wrong, and carry them over to this detailed description. Similarly, no description is detailed enough to get precisely every feature of a face correct (see artists impressions from eye-witness descriptions), so there may be some issues there.
4. Eve can assume the form of a humanoid creature that resembles an arbitrary dwarf, although not necessarily correct in every detail.
   • Yes (with the same caveats relating to potentially getting basic things about Dwarves wrong due to not having actually seen any examples of Dwarves)
5. Eve can assume the form of a Purple People-Eater.
   • As described, Yes

Which of the following become true if Eve sees Bob?

1. Eve can assume the form of Bob the dwarf.
   • Yes
2. Eve can assume the form of an arbitrary dwarf that does not necessarily resemble Bob.
   • Yes, though if Bob is the only Dwarf Eve has seen, they may get some details about arbitrary dwarves wrong. For example if Bob is a male dwarf Eve may get details about female Dwarves wrong, or if Bob is unusual for a Dwarf in some way, Eve may think that unusual feature is standard and bring it over to their arbitrary Dwarf form.