As this is simply a service gesture, not any form of required acommodation, don't count on the next agent permitting the departure airport change at all.

If SWF works better, grab it immediately, ask about HPN once you have SWF locked in and be done with it. Chances are that you will get a solid "no" unless you want to pay the change fees + fare difference, but it does not cost you anything to ask.

Only you can evaluate the risk vs. reward of flying on a smaller aircraft which is more likely to misconnect / cancel and ruin your entire trip, vs. taking a couple of hours off work.

Voucher for a bump? That is not even worth thinking about now. DL may well overbook the flight, but overbooking has nothing to do with overselling. More often than not, inbound misconnects make up for the overbookings and there will be plenty of seats.