If the cone still moves freely in the gap and when you take the cone out, the gap is equal all of the way round then the magnet is fine. Make sure you clean the gap (some kind of sticky tape is good) before reconing in any case.
Personally I use Evostick or similar contact adhesive for attaching the spider and suspension to the chassis. Again, you need to clean back to bare metal before the recone. It's the cleaning up that takes the time, not sticking the new cone in!
For sticking the dust cap on, I've been using UHU general purpose (the yellow bottles/tubes) for cone repairs which seems to work well.
Other people will have their own preferences, some use epoxy instead of contact adhesive. I like evostick because it's quick and easy but you do have to make sure you get it right first time. Not difficult if you have put the shims in the gap around the voice coil.
