This is the thing.. Jet2 have already made it known they would like to get in to LGW, so unless anything has changed those issues you highlight (with the possible exception of slot shortage) are clearly not insurmountable.
Agree regarding SEN. Seems far too close to STN, although with the proximity to London stranger things have happened. Still, Jet2 employ a cautious approach to growth and this is one of the many reasons that they have done so well so far. You don’t generate revenue the way they do by competing with themselves. If they are to pursue growth at smaller airports then I think there are a few others they would look at first.
Neither easyJet or Ryanair took a negative attitude towards basing aircraft at SEN. They believed they could fill the seats at both locations or they would not have come. Jet2 seem to know exactly what they are doing and as Covid memories fade smart companies will want to take advantage of opportunities that are out there before someone else does.