Depends on the situation. In situation 3, if all those things are factual and known, then for me it's a non-roll. Guaranteed to succeed. Otherwise it would depend on what's going on in the game. Are the guards on alert? Drunk? Etc.
I'd start with a harder baseline (say 15-18 since guards aren't in the habit of handing out keys), and adjust from there. On alert? +7-10. Drunk? -5.
If the situation renders something impossible, then it's a non-roll and I explain why.
So not everything is a roll, but things that have a chance of success or failure (at DM's discretion) should be given a chance at that success or failure. Just like adjudicating anything else (I want to tame this wolf; I want to jump out of this 2nd floor window; Etc.)
All I'm saying is I don't like factoring in the player's ability (or inability) to perform those tasks in the outcome, as that's unfair.
You’ve just shifted the burden of persuading the guard via roleplay to persuading the dm via contextual reminders which is less cool.
Putting together basic facts does not require you to be a charismatic person. The problem with relying on checks is you create situations where NPCs behave according to the Charisma stat of the PC they’re interacting with rather than the context of the conversation. Which is dumb. NPCs should not act against their own interest because they’re talking to someone who is not charismatic. And it should be exceptionally hard to get people to act clearly against their own interest.
This also creates a bad habit where DMs make people roll for trivial skill checks where they have a 95% chance of succeeding and a 5% chance of a fun but dumb exaggerated failure. It also encourages the non RAW interpretation where players roll HUGE numbers on trivial tasks and DM think that means they’re supposed to succeed by wider margins.