As of this coming Wednesday, the Frozen singalong will be in the Hyperion Theater, site of the departed AIE, so even closer to TSMM.
With the size of the theater, a FP+ will certainly not be required, but I'd still keep it unless you have something else you want to use it a FP+ for. As a PP noted, your 11:05 - 11:20 means you are being seated for the 11:30 show. No need to get there early or even right at 11:05.
It really is a fun show, with very funny "hosts" and lots of positive energy. I love to watch the little girls who know every word and gesture.