Here’s a link to a master thread on abbreviations etc. - many are for Disney World but a lot apply to both:
I can’t speak for March, but when we went in August it was no trouble to get back in. There’s a security check to get into the area and then just scanning your ticket at the gate of the park you’re entering. Around 5 PM shouldn’t be very busy to get back in so it should be rather simple to do.
If you definitely want to see Fantasmic! then I’d probably say yes. One of the highlights of our trip was eating at the Blue Bayou (w/o the Fantasmic! pass) so adding the pass likely wouldn’t be that much more, at least if your group is eating expensive entrees (we all had the Filet).