I've not seen any rules for lost limbs in 5e. It looks like you'll have to house-rule stuff for this. Both of those solutions can help off-set this Drow's disability, but missing an arm is a big deal.
There are a lot of effects that losing an arm could have. What could it mean for skill checks (rock climbing 1-handed? Good luck!). Bows? Never again. Crossbows? Maybe. Two-handed weapons? Nope. Shields? Pretty much out-of-the-question. What about the social implications? Some people may dislike him just because he's "a cripple," while others may be more understanding.
Benefits of having lost an arm? He weighs less. Kind souls may pity him more. If he's a lefty now instead of a righty in a righty-dominated world, people may be super thrown off by his fighting style. A one-armed man on a battlefield? Likely lower priority than other fighters.
Considering all this, your proposed amendments may work out well. A +2 bonus to hit is pretty big, especially at low levels, so I may consider lowering that to a +1. Overall, though, I still think the drow made a poor choice, and is not coming out ahead after loosing the arm.
Of course, this opens up roleplaying opportunities as well. Rewarding the player for acknowledging their character's disability in various situations is a good idea!
I want to start out mentioning the Cloak of Displacement. It's supposed to mess with sneak attack. Going back several editions, it's done a great job of it. Even thematically, it's very hard to attack a vital point when you effectively have double vision against the target (likely more like multidimensional double vision, but I digress). I don't think you need a way to "fix" that the character couldn't have snuck-attack.
With that said, we're currently utilizing this house rule in a game I'm playing, so....
Generally, it doesn't affect overall game balance
In my opinion, it's less fun, because it's more bean-counting. It very much reminds me of editions from years past where you frantically tried to add various numbers together in order to get to the magic number you needed.
"Okay, I rolled a 7, plus my 6 to hit. That's 13. Wait and I have the plus 2 from charging and another plus 1 because this is my favorite weapon. That's 16. OH OH OH Bardic inspiration! 17!"
Maybe you liked that, but I did not.
In most cases, it's a moot point because you only have one source of advantage and disadvantage anyway. In the rare case that it actually matters and you get to apply [dis]advantage in a case where you couldn't not have otherwise, it's no more or less exciting than any other scenario where you would normally get to apply [dis]advantage.
If this is something you're thinking of instituting at your table, talk it over with the players and come to a consensus. Remember that D&D is, by and large, a bad reality simulator and should not be viewed with too fine a lens, lest you see the frayed threads within.
Best Answer
It is probably balanced
There is actually precedence for this in the variant rule for Lingering Injuries on page 272 in the Dungeon Master's Guide:
These injuries are intended to provide either a neutral or detrimental effect, so the game designers, at least, see it at as balanced.
In my experience, ability checks that involve Intimidation are prompted by the GM much less frequently than other Charisma checks, but that will vary from table to table and from game to game. Overall, I don't see a balance issue with your proposition.
All that being said, it is ultimately up to your GM whether to allow it in the game, as there are no rules to cover starting with injuries in the rulebooks (and the Lingering Injuries rules are optional).