Delta has far better survival odds than Allegiant! Despite its problems, Delta is very likely to be flyong at the end of May when you are going, and given that BOS-MCO is one of their most important routes, they will certainly be flying it. Allegiant has a very short record, and its predecessors on the Portsmouth or Worcester to Sanford routes have stopped business every now and then, and cancelled the occasional flight when the planes was not as full as they wanted. It's just not a very good record. Also, with a major carrier like Delta, you will have a better likelihood of another airline hororing your ticket; on a charter carrier like Allegiant, you have no such probability.
It just strikes me that using Allegiant as a backup for Delta is like using a bald tire to replace one with somewhat worn tread. If you are really interested in a backup plan, think about a car reservation, one that can be cancelled at no cost.
Good luck!