Just something to add regarding seat locks - the 3 year seatlock expires after those 36 months and at that point if you've not been afforded the opportunity to upgrade you can then switch FO seats. If you choose to do that, I believe you're now re-seatlocked for TWO years and that includes upgrading. Everything else is correct AFAIK.

Resigning seniority - that can't happen until you accept the position and start class. They're taking it seriously and they expect you to do the same if you're interested in the job at XJ.