OK thanks for the replies. Yes I bought very short (100mm) cranks since that were shortened from from stock cranks since I have a knee flexion problem. It sounds like the undertightening is probably not a problem, although perhaps there is some weird interaction with the helicoil.

Do normal aluminum cranks have some kind of steel threaded insert? I think that would be normal since aluminum is perhaps too soft to have a steel pedal threaded into it.

It basically seems like the helicoil did self-destruct. Half of the hole is smooth and the coils came out in pieces.

Also I used white lithium grease on the pedal threads which I believe should be fine.