I was scheduled to fly FCO (Rome)-BUF via EWR. My FCO-EWR flight, scheduled to depart at 10:30 AM, was delayed and then canceled due to mechanical problems. Fortunately I was able to be put on a later FCO-EWR flight leaving at 1:00 PM (2 1/2 hour delay). However, due to the delay, I missed my EWR-BUF connection, so had to take a later flight, arriving in BUF about 5 hours late. I very quickly got a "Customer appreciation" email with a link to a page where it asked for flight info. I filled that out, and it then offered me a $225 voucher. (Note: I am 1K).

My questions:
Am I eligible for the maximum EU 261 compensation of €600 since my flight was more than 3500 km and I arrived more than 3 hours late at my final destination? Or does the "3 hour rule" only apply to the initial flight out of Europe (which, again, was canceled but I ended up only being delayed 2 1/2 hours on that leg)?

If I accept the $225 voucher, am I giving up further compensation under EU 261?

If I am entitled to more compensation than the $225, how best to go about claiming it?

Thanks!
DanK