CR of the enemy force is not meant to be compared to a party's CR, it is meant to be compared to a party's APL (average party level), after adjusting for larger- or smaller-than-normal party. In this case, with 4 PCs of level 19 and 1 PC of level 21, your party had an effective APL of 19.4.
Bargle and Azazel are each CR 20 opponents alone. The fact that Bargle had to use a standard action to get Azazel there doesn't justify ignoring Azazel's contribution as a "summoned" creature -- and I don't care if the RAW say that's what you should do. Bargle might as well have spent a standard action to activate an alarm of some sort calling in Azazel from the next room; the effective challenge to the party would have been the same, except that Bargle is out a feat this way. That missing feat doesn't even begin to balance out Azazel's contribution.
Plus, by my reading of the Advanced and Giant templates, each of them bumps the HD of the creature by +2, so Azazel was a 22HD creature -- beyond the 18HD limit of True Name. That should seal the deal. Azazel counts as an additional CR 20 opponent, giving the overall encounter a CR of 22.
So, you've got your 19.4 APL party facing a 22 CR encounter. That puts it between the "hard" and "epic" difficulties for the party, according to http://paizo.com/pathfinderRPG/prd/gamemastering.html And you're doing this when, in your words, they've already expended "most of the PCs resources".
Yeah, it should have been tough. It should have been borderline "epic" even if they'd had most or all of their resources to bring to bear. The fact that they lived only through luck of the dice doesn't in any way indicate that SR isn't good enough with rules as written -- it just shows that they were in over their heads by taking it on after a "grueling stage".
With all that said, I wouldn't personally be against house ruling something for SR. But I agree with Morgen, this is not at all a good example to show the need for it.