We are not going to cover all these allegedly unfounded accusations against four aforementioned devotees right here, are we? ISKCON's policy is not to talk about them if possible and I think it's a very sensible one.

There isn't much sankirtana spreading out of GM temples in Mayapur nowadays, is there? They can't honestly claim this adbhuta-mandir status now even if it appeared so when Srila Bhaktisiddhanta was present. Nor do they have people from all over the world come and chant the Holy Name. Nor do they look "adbhuta" next to TVOP.

This is the exact same thing that was said about Bhaktivinoda Thakur's discovery of yoga-pith - we already have one and it's not there!

And I just don't understand the point of this accusation. What do they expect ISKCON to do? Not build a temple? Deny that it's the biggest temple in the area? Even the existing one kicks ass. Tell all the visitors to go to GM instead? Tell them about "real" adbhuta mandir? That small empty one down the road and not this new one that dwarfs Prabhupada Samadhi, which itself dwarfs any GM temple there?

That thing is huge and as adbhuta as they come. How could anyone deny it?

And who gets to decide that "fall-vada" is apasiddhanta? What's their authority? And why do they select only quotes that suit them while ISKCON shows quotes from the same books in support of "fall-vada"?

Prabhupada was very clear about it and yet these people (some of them) accept opposing conclusions while professing their loyalty to him. Bhaktisiddhanta Sarasvati was clear about it, too. There could be ambiguous passages in Bhaktivinoda but there's support for fall-vada in his works, too, as I said. Even Sanatana Gosvami mentions jiva falldown in Bhagavatamrita.

On whose authority should we reject all these acharyas and accept some other siddhanta?

The OP was about guru-tattva. Do we really have to discuss all those other things? It's like people carry huge chips on their shoulders and can't wait to unload them on anyone who'd listen. How did you get caught up in all this?