the only time this setting actually is faster is in see ya ending because you have perfect timing on the golden freddy hide (which is also possible without it), otherwise as turku said it can often be slower and isn't really worth it

lmao i've been routing all achievements for weeks ;0;

there are five! there's one on night one after you get the screwdriver and one on each subsequent night :)

ggs!! huge run

the "No Major Skips" subcategory is what you are looking for. some minor skips and glitches are technically allowed as long as they don't skip large areas of the game or go out of bounds. you can see what skips are allowed in each category in the guide document in the guides section

the head is not necessary if you skip west arcade because after you kill Roxy and leave the boss fight it sets the time to 5:40. although you are correct, upgrading Freddy with the eyes does skip it to 5:50AM because it's based on the upgrade itself and not the number of upgrades you have

there's a beta autosplitter for the Vanny ending available on the discord server, and I'm sure the community modders are trying to implement it for other endings too. hope this helps