1. you shouldnt have any die-off if you dont keep them out of water for any long amount of time. just transfering rocks to another tank, you should be fine. the only thing that might be hurt is a sponge. they dont do well when exposed to air, so if you have them you might want to do your best to keep them submerged. those pieces you might want to keep in the main tank.
2. adding dead rock will be fine. just rince them thoroughly with saltwater and try to get off any dust or debris. there will be nothing living on them to die off, and as long as you rinse them well, you should be okay. keep an eye on your parameters for a while after they are added just to be safe.
3. if you want to use the show pieces upright, you can do that. i would lean them against a supporting structure behind them, not just rely on gravity and the sand bed to hold them upright. another option is to build a supporting structure out of pvc
and use that to hold up any rock you want elevated. then you can use smaller pieces to fill in the gaps, and cover any exposed pvc
you might not like. a nice treat is that you can build a good size cave for the eel and cover it with rock, rather than chipping your rocks to fit together secure enough so the eel wont knock them apart.
4. long posts are like buttered bread landing buttered side down... it is inevitable.