How to define and classify "Progressive Rock"?

Haha..! Well, that's a problem for you, Collaborators. You have your standards/instructions and/or sub-genre definitions that may be your guidelines. But your decisions do not necessarily meet with an approval of the "general" public, and the genre (or sub-genre) attributed to an artist may not be satisfactory to everyone.

"We agree more than we disagree" is a much weaker statement than "there is consensus". Just saying.

I'd say that consensus is constituted by agreement, rather than being a cause of it. You have consensus to the degree that you agree, and no further. By the way, as Prog Reviewer I can watch the team threads and disagreement isn't that rare in my perception. Yes, you agree more than you disagree, but there's enough disagreement to find the statement that there is overall consensus dubious (I'm not specifically referring to teams of which you are a member).

Generally I think that you miss the fact that pragmatic use and degrees of agreement are not criteria of truth or correctness. Yes it is useful and pragmatic to have well defined genres and in a team it is useful to try to agree on genre definitions. The PA definitions result from a long negotiation process and have been changed at times. There are some inclusions and exclusions with which a number of collaborators don't agree, and things may change in the future. The PA definitions are pragmatic to a certain extent and they are agreed to a certain extent, as the current manifestation of the negotiation process that may go on. However they do not refer to any objective truth; they are constructed in this way not because this is the only correct way of doing it, but because something like this is needed, so some agreement should be reached, which could as well be different, but now it is like this.

The current definitions can be defended on pragmatic grounds, but they are ultimately not binding for anyone outside the process in which they are used, i.e., for accepting, rejecting, and categorising PA entries. Who does not take part in this can well have different personal definitions (or refer to different places where there are different categories); actually this even holds for collabs as long as they are not specifically  involved in such decisions, and in fact even if involved in such decisions, as disagreement may be rare but happens.

(Though I'd probably agree with you thinking that if somebody comes up with some ad hoc genre definitions and claims that these are better than those of PA based on the odd decision that they don't like, we can suspect that this person underestimates the thought, effort, and controversy that has gone into the PA definitions and may well underestimate their power.)