about last boss, if youre getting sentry droids youre doing it wrong. click all 4 terminals *at the same time*.
we've also finished vet mode with 230 geared group, so i'd look for the issue elsewhere, believe it or not, healers also have AoE heals so that they can heal both, the tank and other people at same time.
and if tank cant keep aggro its either because the DPS pull them all around the map (again not problem of FP) or the tank AoE taunts too early / doesnt use his AoE attacks at all, but still, 1 stray mob will definitely not make issues for a half competent healer either way.