The fact that the addictional terms are basically designed to defeat the original licence should make it clear to anyone that it isn't a "reasonable legal notice".

IMO this is a great illustration of how the word "reasonable" stops the entire legal system collapsing.

This all seems a bit silly. Sneaky Company, Inc. can also add in that this new clause should be removed.

The problem here isn't the legal interpretation of the license and adding new clauses doesn't seem to do anything useful. Sneaky Company can license their software under whatever weird combination of terms they like. If they want GPL except it's proprietary then they can do that. Weird but ok. Same again if they want the license to be internally contradictory - seems stupid but they can do that too.

The actual issue is if they're trying to be deceptive by framing something as GPL licensed when in fact it is some new not-in-the-GPL-spirit license that they have developed. They should either be generally shamed or sued by whoever has copyright over the term "GPL" (pretty sure that is the FSF). That'd be ironic but there you go. Or some sort of false advertising suite.

First time I hear about Euro-Office. I wonder if that's a serious project or just fishing for EU funding.

Quite clearly OnlyOffice never wanted to have a fully free software office suite. The (mis)use of AGPL has always been a marketing trick and nothing more.