Late winter/early spring is the best time. That's what it is around here right now (zone 3) so I just pruned all my fruit trees this past week now that the snow is mostly gone and the buds haven't yet swelled. I take it you are further along in the season, but you can't be leaps and bounds ahead so I think you would still be near the best time. Late fall would be the worst time since the trees would be going into winter with fresh injuries from the cuts and that could be kind of tough on them.

Yes, now it's good, ..cuts can heal in faster now when growing.
Cut branches as close as you can on the trunk, you might need to cut some other branches out to get good light in, [you get tastier apples].. and might need to shorten some branches/ends a little.

The tree folks will warn you about flush cuts, although crabapples are strong trees and usually heal their wounds well no matter how you make the cuts or when. Still I would leave a bit of a swell from the trunk so as not to violate the branch collar (google Alex Shigo, branch collar for details).

If you wait until the trees have been in leaf a couple weeks (and have built up their reserves a bit) it will be less stressful, supposedly, than pruning during first growth, but a vigorous, mature crabapple should rebound pretty much whenever you prune it none the worse for wear. They heal even better than a healthy apple tree.

I'm guessing the serious folks like Brandon would give you much the same info at the tree site.