X1 Search 8 is long out of beta. Which version do you actually have?
What on earth makes you use that wonky scenario? A USB memory stick is asking for trouble. Windows enjoys losing them. Is there any reason why you don't use an internal drive?
See above.
I have 30GB of PSTs in 10 files and don't see any problems.
But I do run a slightly eccentric setup. I attach most of my PSTs to a secondary Mail Profile that has no related mail accounts.