I think Jet2 rightly placed an aircraft in ALC to provide a rapid response should they have a tech issue in Spain, it is always going to be quicker to resolve that with a standby aircraft nearby, of course some of this was probably in response to the care levels that are expected (and protected) with inclusive tours holidays.
With regard to the share price, well they go up and down just the same as everyone else, as this season winds down all eyes will be on next years forward bookings, if the pound remains weak in to early new year people will likely hold off committing to multiple EU holidays and with Sharm, Tunisia and parts of Turkey off limits, then many will only look at EU destinations or the states and Jet2 can't really do the latter with very old 757's up against operators with 787's other than Xmas specials.
Brexit and 25+ new 738's are a toxic mix for an airline that is very seasonal in its operation, 2017 will be a challenging year for all airlines, aren't they all! but thus far things have worked out for Jet2 and i suspect that they will again, they need to replace their older aircraft and i'm sure they will have got a good deal on run out production of NG prior to the MAX, but out of the box the max will fly faster, higher and use less fuel, be it FR with 200 seats or Norwegian flying from Spain to the UK with a Spanish cost base.
Jet2 should size the moment and become a true pan European airline whilst a EU accepted AOC allows, the new 738NG are ideal to base in LPA/TFS/FUE/ACE flying to Scandinavia in winter, good use of the asset is the secret to good returns