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But however insignificant it is, wouldn’t it always be there sapping angular momentum & over a long period of time drag any two objects closer and closer together? What would be the repulsive force counteracting this?

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It would be. Which is why any pair of orbiting bodies will eventually collide.

It’s just that for black holes this effect is insignificant (a merger would take much longer than the age of the Universe) until they get close to each other, much closer than 1 parsec.

So the question isn't how they merge, but how they do it so quickly.

Yes, dynamical friction can "quickly" (on the order of 100 Myr) bring the two black holes to a distance of ~1 pc to each other. Gravitational wave radiation can "quickly" cause a merger if they are within a bit less than ~0.1 pc of each other. But how they go from 1 to 0.1 pc on a timescale of ~100 Myr is unknown.

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