Hi and welcome
1) I think the mezzanine should be ok
. As for open stairs, my in laws have some and even Chaos (ataxic) doesn't fall through
(Onyx once fell through at my parents' but he was rolling over on a step
).
2) It doesn't seem necessary for the enclosure...
3) Mine use one litter tray (I have 2 cats) but if you have room enough, 2 will be better
.
4) My Bengal is one year old and still pretty full of mischief
5) For the harness, start as soon as you can. Onyx started at 2 months old, Chaos at 4.
6) Will they get out ? Because if they don't, a collar isn't necessary...
I don't like collars personally.
7) No precautions
I got a 1,6m cat tree at first for Onyx when he was 2 months old and it was ok
(sadly, they broke the upper part last week
)
9) I don't know this brand of food so can't say. But at first you should begin with what the breeder gave them alse you'll have some tummies upset...
10) As for furniture, it depends on your cats... mine don't scratch the couch but the wallpapers (of the "soft" sort) are to be redone