Does Boomkin Form Buff stack with Demonology's Demonic Pact?
yes

Spriest doesnt bring much to the group outside of VT, but you'd have innervate anyway.
Holy priest has a few very good smart heals, and holy nova is vastly underrated in 5s.

https://www.icy-veins.com/wotlk-clas...fs-and-debuffs