Next, I'd argue that culture-shifting to something that's not near to your starting location is very non-trivial. So much so that it's almost not worth considering, unless you're doing it for the memes. (Eg: Forming Prussia as the Mamluks vs forming Prussia as Poland - both are annoying and expensive, but one is just brutal.)
Depends how many full cores you make. There's a reasonable argument to not spam too many (easier to de-state for institution embrace, multiple drifts, getting absolutism quickly, etc) and thus the sacrifice for switch is smaller.
If you're trying to min/max mission rewards and whatnot, you're also more likely to be expanding fast enough that these things are actually attractive (and full cores less so, since the autonomy difference won't be as pronounced). Unless it is 100% intended as a meme game from the start (always Sunni in Moortugal!).
I believe that this relative uniqueness of missions, NIs, events, starting position, etc is what drives the replayability and success of EU4.
Quite likely, but I would argue that NIs are the smallest component of that equation, because they're the least interactive and players have the least agency over them. In most cases they just make you do some things you're already doing anyway marginally better...they don't drive decisions the same way as missions or even idea group selection tradeoffs might. Also, depending on how you implemented an alternative, start position + game state might mesh into NI replacement in a way that's more interactive than present, with outcomes still very likely given particular geographies or alliance situations etc. There are ways to make Britain-like NI outcomes that Britain itself is likely to get a lot of, without having "British national ideas" in the game.
IMO that would have been the better implementation, but I also acknowledge that I'm talking about a ~10 year old mechanic that existed alongside other mechanics that are long since archaic but was not itself broken...not a strong candidate for a rework. And if you offered me better UI, a better system to model AI willingness to peace out to replace "length of war plus other modifiers" etc I'd pick those in a heartbeat. IMO NIs are bad in terms of current EU 4 design, but they're not devastating and they don't hinder gameplay or anything like that. I'm not convinced they merit a rework in EU 4 even, though I'd be disappointed to see them in EU 5.