Hi Ginny
It is very unusual not to be able to take up the parking offer but I think in this case there was simply no availability. I hope it won't put you off flying from Gatwick in future.
As for your other question, I wouldn't be overly worried about flight times changing. It really doesn't happen that often, but if you want to be absolutely sure I would wait until you receive confirmed flight details with your Final Information, which usually arrives about 2 weeks prior to departure.
Kind regards
Olly, your editor