Defeating the enemy with spies

[Deus ret.]

But then again, perhaps I should do just that, considering I resently won three major battles and they would take a while to pose a real threat again. But on the other hand - the clock is ticking

It is indeed, so just don't do it. You're playing GA in Late, and if I got you right your situation doesn't ask for tactical victories which can buy you some time for the next couple of tactical victories on another front. Now that they're weakened, go ahead and crush them utterly ASAP. Don't relent until this is done, however costly it might be, and don't care about possible reemergences since they will start over with 0 points. You likely won't stand a chance without taking decisive action now.

As for the Spanish, deal with (=exterminate) them any way you like. Depending on the overall scores probably nothing short of annihilating will do the job, so you could try the assassin variant I suggested (difficult to pull off with many BFs) or else wage an all-out war against them once you've finished the Russians. Even if the last remnant of their royal line isn't extincted until 1452 you'll have secured victory, but given the standing and the date you'll have to be committed to some kind of all-out effort to overcome them.

If all this isn't possible, as a last resort you could go on an insane conquering spree against whoever is in range and doesn't really pose a threat on the military side of things. This way, you might secure at least a domination victory before the clock runs out. Admittedly that's somewhat cheap but won is won