Rulings, Not Rules
Jeremy Crawford, the lead game designer, called the rules "intentionally silent on these corner cases":
Wild Shape can introduce wild situations. What happens when someone swallows a druid in a Tiny form? Is a druid fecund in beast form? The rules are intentionally silent on these corner cases, leaving adjudication to DMs. As always, I say go with what's best for your story.
The particular ruling (the druid can shapeshift into a specific animal) can lead to a very satisfactory in-game situation. Or it can be utterly boring and devastating, depending on the plot. It is the DM's job to make the right decision.
Aside from the combat, there will always be huge difference between classes' features, so you can't compare (or "balance") them. Some features will be much more useful than another ones in certain situations.
Following the rules-as-written as strict as possible won't help here. In the end of the day, making the game fun an engaging is not about the rules. For instance, if your game is all about picking locks and disabling traps, and you have only one rogue in the party, (s)he inevitably steals the spotlight. You, the DM, have to balance these things by your self — how exactly do class features work in order to not to spoil the fun.
You are the DM and you want to decide, how does magic (the Wild Shape, in particular) work in your world. For this job, what things should you consider in the first place — your own story, the fact if your players have fun, common sense, or nitpicking these minor semantic details in the rules (which are concise and not detailed enough)?
The Adventurer's League Guide describes the role of the DM the similar way:
As the Dungeon Master, the most important aspect of your role is facilitating the enjoyment of the game for the players. You help guide the narrative and bring the words on the pages of the adventure to life. The outcome of a fun game session often creates stories that live well beyond the play experience at the table. Always follow this golden rule when you DM for a group: Make decisions and adjudications that enhance the fun of the adventure when possible.
Wild Shape description is open-ended
PHB gives only basic restrictons of the Wild Shape:
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
Your druid level determines the beasts you can transform into, as shown in the Beast Shapes table.
The only limitations it describes are the creature type, its CR and its flying/swimming speed — pure balancing ones, a sound base for DMs to build their own adventures. As a DM, you are free to apply all the necessary restrictions — the creature size, type, features or appearance. It would be reasonable to discuss this with the player beforehand, say, prepare a list of their wild shapes.
You don't have to though — if you are happy with the default restrictions, just say that all other things are allowed (dinosaurs included), unless it spoils the fun.
Your reasoning seems sound to me
As a wildshaped druid, you:
retain your alignment, personality and Intelligence, Wisdom, and Charisma scores
(PHB, pg. 67)
In other words, your retain your mental capacity and your character's mind, and thus your memory, creativity and reasoning abilities, so there's no reason that, given the Raven's Mimicry trait, you wouldn't be able to "mimic" the sounds you've heard in life to effectively speak (in any languages you know as a druid) as a Raven.
The only way in which this falls down is that some may argue that language doesn't come under simple sounds, as mentioned in the Raven's Mimicry trait's description:
The raven can mimic simple sounds it has heard, such as a person whispering, a baby crying, or an animal chittering.
However, real life ravens can mimic human speech, so what is considered a simple sound vs. a sound too complex for a Raven to mimic, and where the sound of a person saying a word falls into that, is for each DM to decide.
At the very least, I would argue that a single word is a simple sound (simple enough for real life raven's to mimic anyway), and it's the druid's intelligence that can then string those into a sentence, so I would still argue that the fact that the Raven's Mimicry trait calls out "simple sounds" wouldn't stop this from working as you intend.
It's worth pointing out that this differs from Kenku, since Kenku have a curse that prevents them from being able to create new sounds, as they are cursed to be unable to express creativity (as this question goes into).
Best Answer
Generally, yes
First, the grid space of a map does not represent the actual space a creature occupies, it represents the space that creature controls:
A large creature's four-square footprint is an abstraction of its size and generally occupied area. At any given time, it might be located more in one of those squares than the others.
Second, the core process of playing D&D, as outlined in the introduction, involves the following:
In essence, the process of playing D&D is a back-and-forth shared storytelling experience. As the player reverts from a large creature into their normal form, the DM and player cooperatively arrive at how the action unfolds. If the player wants their character to revert to medium size and end up in a specific square, they convey that information to the DM.
If there is some reason why that particular space is not an option for the player to end up in after they revert, the DM would describe the reason for this limitation. But this a unique narrative reason for a space not being an option, rather than a general one for the player being forced to end up in a specific space.
More broadly, the player is free to have the action of reverting place their character in any of the controlled/occupied spaces because those spaces are controlled/occupied by the character.
As for how this reversion affects attacks of opportunities, I'd say that, from a standpoint of practicality, it makes sense to have the Druid end up in the same space relative to the larger form as when s/he changed shape in the first place. If the medium form were in the lack left square relative to the polar bear, it makes sense to have the reverted form end up in that same square.
Does that mean they get a "free disengage?" They burned a bonus action to revert, so I'd argue it wasn't free and point to Rogue's Cunning Action, another level 2 ability that allows a character to Disengage as a bonus action.
What about a situation where the druid and enemies have equal movement? The druid is now just beyond 5' range, so if s/he reverts and moves, the enemies will have to Dash to cover that 5' gap and get within attack range again.
Consider that an untransformed Druid is a caster and having enemies in melee with them (even if the foes used their action to Dash) is a very unhappy situation for that Druid to be in. If the Druid uses its action to Dash in addition to its bonus action to revert, they likely have just dashed away from other melee members of the party who could help.
Bottom line, if the DM is so concerned with being able to deal damage to the Druid that s/he is breaking out the proverbial measuring chains, you might have a larger issue to resolve at your table.
I'd say it's better to look at it this way: certain classes get certain perks and them's the perks of being a Druid.