That is not acceptable for me.

Back buttering is only necessary when special situations arise, like odd corners/niches where spreading thinset on the backer is not possible.

I find it much easier to install 18" tiles evenly than 12".

I'd insist on an even floor.

If the lippage is less than the thickness of a dime, it's within industry standards. It also depends on the type of tile--a terracotta tile is going to have way more variation than rectified porcelain. The out-of-level problem is worse, but it could be that the whole house is out of level.

I have a feeling that if you noticed, it's way screwed up. Most people don't notice even fairly large mistakes, in my experience. Also, the grout joints are probably way off.

There is a difference between a level tile floor and an even tile floor. A level tile floor can be uneven, and an even tile floor can be unlevel.

That may be just the difference in words(semantics) but it is often very important.