If someone is hiding, do detection attempts always have disadvantage?

This is a great question. From the rules, the answer appears to be 'No', but it isn't explicitly stated. I'm inferring it from this section of the basic rules for Hiding:

Passive Perception. When you hide, there’s a chance someone will notice you even if they aren’t searching. To determine whether such a creature notices you, the DM compares your Dexterity (Stealth) check with that creature’s passive Wisdom (Perception) score, which equals 10 + the creature’s Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5.

For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Wisdom (Perception) of 14.

Emphasis mine. In this case, the rules are specifically explaining how to calculate the passive perception to detect a hidden creature. Since the rules for hidden dictate that the creature must necessarily be unable to be seen clearly (implying light or heavy obscurement), we can infer that this hidden creature is in some way obscured from the searcher. However, since the passive perception total listed is 14, and does not in any way reference a -5 modifier for an obscured creature, it seems like we can be reasonably sure that detecting hidden creatures happens outside the influence of obscurement. Otherwise, the math present would necessarily have to include a -5 for detecting an obscured creature.

Can you sneak up on someone in dim light according to RAW?

This one is DM dependent according to the rules. We can piece this together from the examples that the basic rules give us:

In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you.

This gives rules for when a creature can detect a hidden creature approaching it. However, since it specifically calls out 'in combat', and since 'out of combat' is not addressed, we have to assume that being out of combat doesn't change the core ruling of the 'hidden' effect (otherwise it would also have a callout, because specific beats general).

The generic hiding rules that necessarily must apply out of combat are:

You can’t hide from a creature that can see you, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position.

and the errata:

The DM decides when circumstances are appropriate for hiding. Also, the question isn’t whether a creature can see you when you’re hiding. The question is whether it can see you clearly.

So in this case, the game is deferring to the DM. You are well within RAW to tell the player that they can't approach the character in dim light from the front and remain hidden. This is even more overt than a simple rule-0, because the published rules specifically call out the DM's ability to overrule it.

My reading of the published rules appear to default to allowing that, but they also give the huge caveat that the DM can overrule it. However, with what we found out above, the detecting creature would NOT receive a -5 penalty by default (unless conditions occurred to warrant disadvantage on the check(s)).

You are correct. If you were in total darkness, that would be equivalent to Dim Light/Lightly Obscured to a creature with Dark Vision. However, you would still have to make a stealth check to hide, as the wording is "You can try to hide..." (bold added for emphasis).

Hiding (PHB p. 177)

"When you try to hide, you make a Dexterity (Stealth) check...."