Well, once you are listed, there is always the possibility of getting bumped, it just depends. We do not have any kind of "lock" on the seat 24 (or whatever) hours out like FDX does. If the company needs to use the jseat for a higher priority jseater at the last minute, they will bump you. You essentially have the same priority as our own pilots do when traveling on UPS acft on their own.
Call the jseat desk number that you originally booked the jseat on the day prior or just before the flight and ask them how many seats are taken/how many are still open. As long as the flight is not fully booked you (should) be fine.
As far as wx/mx, etc..., it's no different than the pax carriers. The main difference is that we generally are more on-time (not waiting for pax as much) and it is not uncommon to have flights leave 15 minutes or so early if the planes loaded and ready to go.
Where are you trying to jseat from/to?