Maybe I missed it, but I've never seen anyone blame Nimoy for the problems with ST3.
If anything, if it's the prop and VFX departments who are to blame for the casket flubs. I'm pretty sure they could have done a better job with the casket - even in 1983/4!
As for the storyline gaffs, one has to look no further than Harve Bennett. Nimoy approved the script and story as the film's director, but it's not like he wrote it!