I have played with these a bit and thought it over and this is how *I* am going to use/rule these plot points and scenario types...
The
Damned Altar seems to work just fine for mobs. 1 mob = 1 base. So, one action by the "mob" of praying is just 1 action for that mob. When they ask for a blessing, it is for the entire mob (all the same, to keep it simple). Seems to work fine and I like the idea of a mob of chanting cultists all transforming... or going poof!
The
Living Idol appears to be written to allow mass worship and that damned-thing is as much a hazard to the Lurker as the good guy! But in this case, one mob is actually 5 figures praying so it adds up quickly. But, then again, I am using MiGo so it works the same way for them anyway.

I may back off on this plot point later after another test run or two and some player feedback form my group however.

The last thing I was worried about that
works this way was
The Devil's Hopyard. This insane crafting of Uncle Mike's is a bit different and is actually a freaking scenario so it is FREE should the Lurker get a good roll. If I allow mass suicides by mobs the Lurker can call some big-ass daemons real, real quick. On turn 1 with a tiny effort. So, I just restricted the plot point to a single suicide per turn. IMHO this is also not hard to rationalize/visualize with it being a true ritual and while technically a suicide it could very well require much ceremony (well, at least one action's worth!), even assistance by a priest (say with a big knife standing over them! Thus...
House Rule for Devil's Hop Yard
Maximum of 1, single, figure, attempting suicide for the "Devil's Hopyard" per turn.
House Rule for the Living Idol and Damned Altar
Mobs may use the Plot Item in which case all references to "figure" are per mob (ex. Re. the bless for The Damned Altar roll one time and apply the effect to all figures in the mob)