Reminds me of Uuno Turhapuro, an old Finnish movie character https://im.mtvuutiset.fi/image/1213728/landscape16_9/752/423/5e2ced3cb480e987cfbfa3845cc7e934/qa/1612132.jpg

A man went to a shop and they had a shovel.

Yes, you have to do some quests first. I think it was around mid or late Act 2 when he got usable as a member. I am not sure which quest, likely the questline regarding the sick man in the Last Light Inn / little boy and his ghost.

I got a quest log message that Halsin had left, though he was still in the camp and was usable in the party. Everything worked as if nothing happened.

Could be a bug.

Sounds like an easy way to lose the keys and forget the thing. Have to always have the keys in the pants or risk forgetting them… at least until the front door gets closed.