In a major investigation the only difference between FAA and the other parties (company, pilots union, manufacturer, etc) is that FAA is automatically granted party status. FAA has no right to interview anybody -- only NTSB has that right, and they don't try to do so without the pilots having representation available.
As to the guarding of the pilots, that was nice of those guys, but not something necessarily unique to ALPA. At the other accidents (as FPA) we did what was required to ensure that nobody had access to the crews before representation was in place. ALPA does provide other help that is invaluable, but nothing that has been mentioned here and is very off-topic for this thread. I just didn't want people to have the impression that our crews were not being guarded on previous accidents.