A: Metamorphosis (and Madness) specifically state that any special powers of the Keeper they are attached to are nullified, so the Feds cannot “cure themselves” of either one. Note that in order to use their special power, you sacrifice the Feds Keeper, so in this case, it makes them seem oddly sane that they don’t want to kill themselves even when they go Mad or start to Metamorphose.

Nightmares, on the other hand, while they do attach, do not impair the Keeper, so the Feds could “kill themselves” to “cure” themselves of Nightmares. This would be perfectly effective, gameplay-wise, though it does seem a bit drastic, thematically:

“Agent Jones is having terrible nightmares, Sir!”
“Well, clearly, the only way to solve this case is to kill him, even if it means taking some casualties ourselves….”
“I’ll assemble a team, Sir!”