You can search the Fediverse from one instance using the Magazines tab in Kbin to find places to sub, or sub to communities you find in all feed etc?
This is the first thing. I think this might not always be turning up everything due to the delays with federation. While we might be able to agree that this is good enough, I think another reasonable person can look at this and say that there's room for some technical improvements.
Is the issue to do with the duplication of communities at present
This is the second one. As others have also pointed out, reddit has the same issue so it's not unique to federation (tho this person seems to get hung up specifically on the precise naming to make it federation specific). I think we can adapt the reddit solution (multireddits) to here as well though to solve this (i.e. come up with a scheme for multimagazines).
But I'm not switching between instances
This is the third one, but I think this is not valid. As you say, one can choose to have multiple accounts on other instances, but it's not needed to participate on the other instances. This person says it's their choice to have the other accounts - but then makes a big stink over the effort of having multiple accounts. Like if it's that much trouble then just don't do it.
long term there does need to be tools to allow communities to migrate base from one instance to another
I thought that this might be an issue but actually I raised this point and it wasn't responded to.
The fourth one is that this person seems to consider kbin.social its own distinct platform - which doesn't make sense in light of federation - and seems to prefer centralization in general (despite seeing the good from multiplexing BBSes), but I'm waiting on a response as to why this should be the case. Like what are the specific arguments to prefer centralization to a single server or a single instance?
It does occur to me however that if a paid shill were to try to promote a centralized service over an open source federated one, a way to win folks over might be to present oneself as a highly experienced technical person with direct expeirence in both kinds of systems, but who ultimately prefers centralization and has good technical arguments to back it up, including pointing out flaws or gaps with the existing federated system. And also insist that more people flock to the single overloaded flagship instance, perhaps causing it to overload and die off.
Not saying for sure that this is the case here, but food for thought.