If you have a first class seat, you can switch to any flight(s) with a first class seat available. Sometimes it takes more than one agent to agree with you, but I've found this much easier and more reliable at the DM level.
Yes, you get the MQM bonus for what you actually fly.
Legal routings are your issue with turning this into any substantial MR. Out of DC, I think the only options to get to NYC are via ATL or DTW. No way they're gonna let you truck it out to LAX or SFO on this one.