Helmet head semi-released - a goner?

nah, just make sure the cotys are developed... sometimes this happens on its own and sometimes it because we didn't bury it deep enough or tamp down the soil enough for it to be pulled off as approaches surface. a couple tiny tweezers goes a long way... make sure you are firmly and only hold a shell then carefully pry it open without putting torque and stress on the plant itself. so the angle you hold it and such helps avoid that. it'll give then can slide off. Look for any membrane holding it back too. little thin layer of 'whatever' might still restrict the cotyledons. that usually slides off too, don't try to rip that one. lose a coty, not too big of a deal... lose a growth tip and it may die or it could be a few weeks before a new growth tip is visible. i.e. don't damage the growth tip! LOL be gentle.. if it seems too forceful, try a different approach or angle of holding it... you want all the pressure/force contained to/applied to the outer shell and not transfered to stem or cotyledons. it's fairly delicate at this stage.