is Severius2 best built in Faithful Masses?

Some thoughts. Briefly, I feel that Sevy2 is best built into both Creator's Might and Faithful Masses, for different reasons. He is weaker into Guardians of the Temple and Exemplar Interdiction because he offers comparatively little to help them overcome their inherent challenges as opposed to other 'casters.

Hand of Vengeance does not apply to light Warjacks. So Blessing will be his normal self in Faithful Masses. Still an auto-include, in my opinion.

Faithful Masses does much for the old man. It provides his heavy 'jacks a boost in hitting power and accuracy, which they *greatly* appreciate. This helps Sevy2 overcome tit-for-tat games. As Sevy2 also favors Idrians, it is not terribly challenging to get HoV to trigger on the heavies, provided you play forward with your Warjacks and back with your Idrians. The wall is nice, though with the abundance of terrain that I've seen in SR2017, it is not the difference maker it once was.

I'm not crazy about the old man in Exemplar Interdiction. Blessed ranged weapons are awesome, but to capitalize on that you will be sacrificing free points from Exemplar models. Also, Sevy2 has Hex Blast already, allowing him to strip most DEF/ARM increasing spells. Sevy2 has few spells that really help Exemplar models out, so you have to turn to list building to shore up weaknesses. You can take Idrians in EI as well (they are Mercs, after all). But that really does cut to the question: why are you running this theme if you are only sort of capitalizing on a couple of things? I'd probably look at Sevy1 for this force before Sevy2.

I also think Creator's Might remains a competitive and viable way to build Sevy2. His Field Marshal is among the best in the game and he has little that supports infantry, trending him towards a battlegroup-heavy force. CM gives you a bunch of free points for support you want and provides Reposition 3 on your Choir and solos. I play against a lot of bouncing effects (Ashes to Ashes, Chain Lightning, Electro Leaps, driving blast damage, etc.) and can tell you that this ability is a godsend. It keeps your support alive and allows them to play forward. Plus, now you can take Idrians in CM. The free points for CM and FM average similarly, in my builds.

The Revelator, on the other hand, is money with Sevy2. Oracular Vision is really freaking annoying on a Colossal who can remain at charge distances. Awareness is excellent coupled with a bevy of guns that ignore Stealth. RAT 5 still sucks, but what are you going to do? And Sevy2 can control infantry like a boss, making them of little threat to the big baddie. I'm picking one up as soon as the holidays are over.