Lemmy.ml, the most popular instance fo Lemmy out there, is not accessible in my country unless I use a VPN. But, I can subscribe to any community that exists on lemmy.ml from here and even post on those communities from this account.
I love this aspect of Lemmy.
This will be nearly impossible. Popular instances can be blocked but someone can simply create a new small one each time. It is eternal cat and mouse. Censorship by the country where it is hosted however will be effective for that instance but no other.