Jack of All Trades and Reliable Talent should work together.
This is also implied by the Proficiency Bonus rules (PHB, p. 173), which state:
Your proficiency bonus can't be added to a single die roll or other number more than once. (...) Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply ...
Mechanically, the requirement is to share a known language
Look at the kenku player's character sheet and the stat sheet of the enemy. If at least one language appears on both sheets, the mechanical requirement is satisfied and the player can use their feature (provided all the other requirements are met, e.g. the enemy being able to hear).
Notably, the ...
A Rogue cannot use Fast Hands on any magical potion
The following quote directly addresses how this feature is limited:
[...] If an item requires an action to activate, that action isn't a function of the Use an Object action, so a feature such as the rogue's Fast Hands can't be used to activate the item [...]
- Dungeon Master's Guide (page 141: ...
One third of the levels of each
In the multiclass spellcasting rules in the Player's Handbook (page 164), the Arcane Trickster and Eldritch Knight are mentioned as counting one third of their levels in Rogue and Fighter, rounded down.
You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and ...
You've done all the math and that's correct. Of course, you need to be using a skill check in which you're proficient, but you are a 20th level PC and built it in such a way to maximize the skill check possibilities.
Don't overthink it. The surprised state ends when the DM determines it ends - the narrative at this point has precedence over stark combat rules, here in this grey area just before and just after initiative is rolled. The DM, if he is any good, should consider your description of your stealthy approach, mitigating environmental factors, the rolls you made ...
This question now has a new answer!
On November 4th 2019, the Class Feature Variants Unearthed Arcana was released.
This UA includes a variant rule for every spellcasting class that has cantrips, that allows them to swap them out, either on long rest or on level up.
Bards, Sorcerers, and Warlocks can now swap them out on long rest: