Act 4 Question

IIRC it's not your level but the area level of the temple for the level 75 thing.
But you wont miss out on anything.
Another IIRC here but you gotta kill him in an area level 75 temple to unlock the Atziri fight.
Kill before area level 75 don't unlock the Royal Access chamber. It's been about 2 months but this is how I remember it.

The architect is a repeatable boss inside the temple. I'm not sure about the level requirement, but you're not inherently gaining/missing anything by fighting sooner or waiting longer to fight him in your temple.

Fighting him destabilizes (deletes) a bunch of rooms all at once in your temple, and every time you fight him, his room moves to a different location in the temple.

He also drops a guaranteed medallion each time you kill him. This includes (but will not always be) a medallion that increases the total number of crystals you can use to run your temple by a factor of 6, each time you get it, up to a maximum of 60. This allows you to run your temple up to 11 times in a row without having to farm more crystals.

Best I can recall, 75 may be the earliest that you can gain access to her chamber after beating the architect and placing/accessing the key room.