I'm pretty sure that in your example w/ SCH, that if a target dies to a helix while unclaimed, you actually get no loot. :/ You do still get kill credit towards vorseals IIRC, but no drops. Been a very long time since I ran into this issue myself, so might not even get vorseal/kill credit.

The error message you saw occurs any time you attempt to initiate any action on a target that is still targetable client-side but already dead server-side.

This is all Working As Intended, and they don't give refunds for items. Ever. ><;;

While Halley IS being a total jerk to you, his earliest advice is actually best. What you are encountering is something that is normally isolated to solo attempts using a pet job, due to pets and masters having claim on different mobs. Bringing a 2nd player to stay on the boss (or clear adds while you stay on the boss) should solve the issue, and GF only scales down to 3x players, so you can bring up to 2x other non-trusts w/o penalty. You really would be better off with a real tank popping and a strat that doesn't rely on SP abilities.

Actually.. you are on Asura too. Send me a /tell when you see me online and I'll grab a couple LS members and knock out a few for you when time permits since I run a LS. ^^;;
You also might want to consider killing the UNMs for the pop items rather than buy them. Now is a great time to do so since double coffer campaign is up, so it's really easy to pile up pop items atm.