Temperature changes. The two materials will expand and contract at different rates, which may be fine or may not. The glass may "spontaneously" break at the slightest touch because it's stetched.

It will be more flexible than glass. I would go thicker if possible.

I replaced a small coffee table top with plexiglass. We've had it about a year, no scratches or yellowing. It's an inch thick, so no sagging either. 42"x28".

You make ice from the waste water of your washing machine? That sounds soapy.