PaulH wrote:
If I file an IFR flight plan 2 hours in advance from departure from a
towered airport and then leave really early or really late:
1. Is the FP normally available to my local tower about half an hour
after filing?

In my experience, typically within 10 minutes or so.

2. How long does it remain in the "system" if I get delayed?

I think typically 2 hours, but I don't think there's any guarantee or
specification. Your mileage may vary.