I can sort of understand the apprehension of a healer DPSing in a DF'd dungeon. However, as long as the healer is good, you can judge just how worried you should be on the first pull alone. That pull takes a lot of trust that your healer won't let you die, so if s/he accomplishes that then your anxiety should subside. I usually tank any DF dungeons I run, so I usually pull 2 groups at a time - if I die from that, then I start pulling 1 group at a time. Healing + DPSing is all about earning your party's trust - but as the OP shows, not every party trusts even the most competent of healers.