When are you going?
I have found that Festival of the Lion King is manageable without a FP due to the large capacity of the theater, but Dumbo moves very slowly! I can't speak to A&E sing-a-long, sorry.
We used a FP+ for the A&E sing-a-long and felt like it would have been better spent elsewhere. They opened the theater to the FP+ and SB lines at the same time, which resulted in the SB people actually getting into the theater first because the FP+ line moves slower due to having to scan at the Mickey readers. I think we would have ended up with better seats had we just gone with the regular line. Don't know if this is standard practice.