This depends heavily on who you talk to, since "fun" is moot here, I'll give my input:
1) Most purists don't use the vault at all, using it isn't wrong per se, but giving yourself many strong artifacts or randarts for your class can very quickly
make a difficulty negligible.
2) This is obviously savescumming and therefore can't be considered fair under any circumstances
I personally feel less rewarded for my efforts when I use the vault or savescum, so I don't do it. If I do use the vault it's usually very sparingly or for a silly build. Some people consider scumming an escort for a necessity in higher difficulties to be fair game, I don't think there's really an excuse below madness for this though.
3) They're just that, quality of life, this doesn't make the game easier or affect gameplay in a meaningful way, so it's fine eg: start in last hope, quick drown, original respec, etc
4-7 are vastly irrelevant as far as fairness goes because it doesn't matter what decisions you take or what items you prepare in the storyline, there's no reason not to roll merchant randarts.
Unless you're abusing bugs or glitches for easy wins (mindslayer snapshotting, stone warden LUA errors) you can do whatever imo. Adventurer is probably also the least legit of all classes to win with depending on how much you intend to break the game
These are just my views on things, but I have played a fair bit of insane and have talked with quite a few people who play insane+