Hey, it's good to know that others have been considering this sort of thing.
My article does detail solutions to some of the issues you've raised here, but I'll go over them each just to see where our visions differ:
I can’t share the post with that friend very easily
All posts will have a publicly available URL. I don't think it would be good to create closed communities, only solutions that would show the user local posts.
If you don’t validate, the system will certainly be abused
I don't believe we should validate that people actually live in the community. I think administration of blocking malicious users should work just like Lemmy, but I don't think the potential for abuse is quite as high, given that the reward for a spammer would be to spam to such a small amount of people. There's less work in spamming to a larger group by choosing just about any other type of community.
Do you have to abandon your old account and start over?
You don't, just like Lemmy and Mastodon, your account on one instance could be used to interact with other instances. The Connecting Instances section of the article details how this could work from a technical point.
It doesn’t have to be one party running this entire system. That’s the point of the Fediverse, right
Distributed cost and administration is exactly how I see it. I would only care to host my local instance.