You misunderstand how Stealth checks work
Of course you can retroactively add to Stealth checks, like any other check with abilities that are in themselves retroactive (like, for example, how shield affects to-hit rolls). However, outside of such things, you can't increase a check total once it's made. Certainly there's no ability that lets you increase your total on a check the next round.
That's not what makes Stealth useful for Theoretical Optimization
You can't increase the total you made in the past, but you can increase your actual result in the future. The important misconception here is that you make one Stealth check while an opponent makes potentially hundreds of Perception checks. In actuality, you make one Stealth roll but many Stealth checks. This is required because:
Both participants in a contest make ability checks appropriate to their efforts
Since Stealth merely has you re-use the initial check's total rather than having you not make a new check, you can totally add to that initial total whenever you reuse it on a new check with anything that adds to checks in general, and nothing retroactive is occurring.
Far more ambiguous is what happens when you then make a third check: is your initial unmodified total used or the recent, higher total? After all, the rules for Stealth say "that check".
The key here is that your second Stealth check, reusing the first total, is also a Stealth check using the same rules. While no rule governs which of two totals you use to contest someone is actually used when there is more than one, that's irrelevant for both practical gaming and theoretical optimization for maximal Stealth checks, for the first case because the Stealth rules do not work as written and are routinely replaced by common sense and even in AL games this particular question doesn't matter since it is only relevant to the Boon of Fate and no player will ever have that and for the second because unambiguously the second check result is used for the third one in at least some sense, even if that sense may not actually have any meaning in game (e.g. if beating the lower of the two Stealth check totals allows a character to make you unbidden and all the other textual stretches necessary to make work arounds not work are also adopted).
Lastly note that that second question regarding Stealth check re-use past a second check only matters for epic boons. For any other ability, applying it a second time does nothing since the first total already includes it and all other bonuses to Stealth ability checks are game features and so don't self-stack. For maximizing effects like guidance, which involve a random component, if you get a bad ruling you just need to drop hiding before recasting the spell, which isn't a problem in situations where you are stealthing across multiple turns and have extra actions, and most DMs will make you drop hiding to cast a spell with verbal components anyhow.