The article feels overoptimistic.

What we can is to create few static motor control connections. If we need to create more, we would need to deal with routing problems - how to make sure nothing conflicts for the same place of 3D space. There is probably something about this in references, but no reference in the place it is mentioned. Or maybe we don't yet know if we can solve this. Also, is there anything to cite about sensory connections in this place of your article?

It seems that some medical operations could temporarily freeze inter-hemisphere communication. It is quite obvious that re-merging the brain should be simpler than coalescence. Did anyone find any human volounteers to tell about the feeling of that?

Also, inter-hemisphere link could have some particular connection patterns which would be hard to reproduce - and without it maybe we need many more connections?

You say that cortex can shift functions around. But that probably uses changing some neurin connections, doesn't it? Can exocortex connections bee mutable? Do we expect the neurons directly on the end of connections to reroute the connections? Do we have any ground to expect or not expect locality problems?

And if brain could be quickly trained to overcome all that, is there any ground to expect anything below complete merge with uncontrolled rate after some threshold of learning to use the connection is passed?

A technical note - "Paths to Coalescence" chapter seems to discuss only one - exocortices. Why use plural in that case?

ETA: I do not know the context and the likely effect of the paper on the people in the context. I just tried to explain why this paper feels optimistic to me by listing the unanswered questions that I had during skimming it (and then rereading part 3 to find out whether I missed the answers).