Lots of ifs and buts Lee, but the answer to your last point is most definitely a yes. They would have priority for the day.
It's potentially a great Trophy draw for them if they beat Aveley, although Aveley have just beaten Hastings in the last round of the Trophy, who in turn dumped Chesham out of the FA Cup!

If they do beat Aveley, the frustrating thing for all at Chesham would be the reduced revenue for what would, in a normal season, be a real money spinning tie. I expect they will do all they can to maximise revenue on the day, pressurising the local council to allow them to offer some form of bar/hospitality.