You post some good questions.
As far as one employee group honoring another's picket line, that is kind of a gray area. I think it boils down to whether your own union formally declares support for the strike or not.
For example, The strike that doomed Eastern involved the pilots supporting the mechanics, but just a few years previous, the mechanics did not support the pilots.
When the AA flight attendants struck in the 90's, the pilots all flew by prearrangement. APA convinced the FA's that more damage would occur to AMR to fly empty airplanes around, but many think that the pilots just pulled a fast one on the FA's on that one.

Replacement workers are always scabs.

As far as crawl backs who return to work for family crisis issues, there will be some who understand, but most will not and there will always be a small but loud contingent who will make a big issue of it for decades.

A scab is supposed to be tainted for life, but interestingly, ALPA officially gave all the Continental scabs amnesty in order to re-organize there.

Another gray area that has come up in the past and may again are the runaway shop airlines. These are airlines set up directly or indirectly by the management of union carriers to be non-union. Thy many times are set up with planes, routes, and assets stolen from the union carriers giving their bosses fits.

New York Air was one of those set up by Lorenzo. Mesa Airlines recently had one of those but it was melded in to the main group after negotiation. People still argue whether those kinds of pilots are scabs or not.