For the fragility of ghosts, I wouldn't consider them anywhere near fragile. The shotgun Lord Felwinter used was likely akin to the one that now bears his name, and any guardians who stepped into the crucible before Oryx's arrival can attest to it's power. At point blank, no one, guardian or otherwise, could survive that unscathed. For why ghosts aren't destroyed in the crucible, I don't think they'd accompany their guardians into the crucible at all. I believe that the ghosts are in a designated zone outside the play area and you're revived there, then whatever systems the crucible employ to secure a zone for play transmat you back to where you're supposed to be, and the "ghost" that is left where you physically died is just an artificial death marker with certain mechanics built into the crucible system. Because even with the rule of never shooting another guardian's ghost, what about if a rocket goes off on top of it? The Ghost would likely be destroyed and then you'd have an actually dead guardian