You're good on the Stratagems, the wording only requires you to have a valid Death Guard Detachment (Mortarion being one) to have access. Though I think the only thing that stands between you and your other detachment being Death Guard as well is the Jump Pack.
This is the only way I can get a csm sorcerer in to try and warp time mortarion. The other power will be Death hex.
Caveat: I think you're overvaluing the Command Points. Yes, stratagems are fun and good, but you've forced yourself into 5 Troops choices - which would be fine if they looked anywhere near effective/useful. What's your plan for getting value out of them? They can initially serve as ablative wounds for a character, but you only need so many cheap objective holders. What if they live through the first round or two?
The bloodletters and bloodthrone are going to deep strike and charge 3d6 in to their lines and cause some havoc. The nurglings are to deploy up front and make a pocket for the blood bomb to get into. Pox and cultists hold backfield objectives. Thus the five units.
I could also swap the prince and cultists for some obliteraters then swap plague caster for a jump Lord to buff the obliteraters. But without being able to double shoot, I don’t think it’s worth it.
You say I’m overvaluing the cps but man this list is hungry for them. The bloodletters and bloodthrone take up 3 to begin. Then if the charge goes well or doesn’t, another 3 lets the letters fight again. Leaving me with zero cp.
So the plan is to hope that people won’t be able to put down mortarion, both plague drones and the prince, if I don’t get first turn.
And if I get first turn, the devastation begins early.