I didn't have time to look at everything, but one thing that jumped out at me was your statement that the dagger of blindsight let you see through the fog cloud. Unless 5.5e changed things, blindsight doesn't let you see through it.
"BLINDSIGHT
A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense."
Tasha's relies on you being able to see the target(s). As written, at least in 5e, that strategy wouldn't work since while you would know where they were, you could not see them. No idea about 5.5e, though.