Apologies in advance for asking beginner questions, if there's a FAQ or wiki page that answers these, let me know.

I have found the elusive "low" level business class roundtrip to Europe (DFW-LHR-DFW) for dates that work for me. But the return flight is less than ideal: a 7+ hour layover in DTW. There's an earlier flight for the domestic connection, but that prices out at a whopping 50K miles extra (150K vs 100K). So I wonder:

1. What if I was willing to downgrade for the last leg? Can I combine classes on a single routing? If there is low coach availability DTW-DFW, can I fly business class LHR-DTW and then coach DTW-DFW for the 50K price of the LHR-US business class low award? If so, would that take a call to DL reservations to book?

2. If better flights LHR-DFW open up in low between now and the flight date, is there a fee to change (changing date/time, same origin/destination)?

3. If I can't get the early flight confirmed, what are my day-of-travel options? Is it just the paid $50 confirmed change? I assume that would keep me in F is space was available, or I could fly Y if not? Any free standby on award ticket connections? Is checked baggage an issue?

Edit to add - I have no status on DL, in case that matters.