Liberalism has an actual definition, and it is not the colloquial definition used in mass-media to refer to "the left half of what is acceptable."
Liberalism is an idealist (another word which has a very specific definition) political philosophy which champions private property, constitutionalism, republicanism, rule of law, and free trade. It has a philosophical canon, flowing through writers like Locke, Montesquieu, Mirabeau, Rousseau, Paine, etc. Further economic works, like Smith's "Wealth of Nations," are built on this philosophical underpinning.
Marxists are materialists. This is in contrast with the idealism of Liberals. While Liberals believe ideas are the force which drives change in the material world, Marxists understand that ideas are just a reflection of the material conditions they emerge from.
Liberals find themselves banging their heads against the walls of the institutions time and time again, because from their perspective, these institutions are just a reflection of ideas, and as long as the justification for an institution on paper is sound, there is no reason to think it cannot be reformed. An institution like the US Congress, or the Executive Branch is never at fault. It is simply a good institution simply being run by bad people. Marxists (and Anarchists) reject this quite simply, by looking at the material incentives involved, and the long ghastly history surrounding these institutions.
"Combating liberalism" does not mean being a piece of shit to anybody to the right of Bernie Sanders or Jeromy Corbin. There is a genuine struggle to ensure the new crop of social media platforms don't simply end up defending the legitimacy of the established institutions at the expense of genuine radicals who find themselves at odds with the actual longstanding policy and practices of these institutions. To avoid situations like when mastodon.lol banned CODEPINK, a prominent anti-war organization, for being "Tankies." This is Liberalism, and it should be combated.
This is not true. ActivityPub (the protocol Lemmy instances use to speak with one-another) does not intend to be a redundant, distributed datastore. There are a few reasons for this. One is practical. It needs to be affordable to start a new instance. If the requirements for starting a new instance entail mirroring significant parts of the fediverse (a network of over 2 million users and 22,000 instances) it would be impossible for anybody to do it unless they were Google/Facebook.
Another has to do with trust. A community has a home. That home is chosen (ideally) because the admins can be trusted. That instance is the universal source of truth for that community. If communities didn't live on a specific instance, they would be vulnerable to various forms of hijacking. The home instance has the final say on who has permission to comment, and who has permission to perform moderator actions. None of these actions could be trusted if they weren't cleared by the home instance first. Third party servers perform basic validataion against the currently known ban list / mod list / etc, but this could easily be spoofed by malicious instances.
When an instance goes down, it is kind of similar to a netsplit on IRC. A queue of outgoing messages build up on your instance, which can be seen on your instance. Queues of messages queue up on other instances, which can be seen on other instances, but they won't be synchronized until the destination instance returns (this depends specifically on which inbox the messages are directed towards - I'm not particularly familliar with the specific implementation in Lemmy).
Finally (though not really), ActivityPub isn't designed to be a broadcasting protocol. In the case of Lemmy, and other Reddit-like clones, it effectively acts as such, but it is intended only to send messages to the places they belong. If you post a message and the subscribers to that message only exist on 3 servers, that message ONLY gets sent to those three servers, even though there are thousands of servers in the network (at least, this is how it is supposed to work in theory).
I might have some details wrong here. I'm more familiar with how Mastodon works (and how it fails) at this point after troubleshooting various problems on my instance.