Theres nothing that prevents Madhouse from employing Togashi or enterting into an agreement with the manga publisher. Hows it any different than an anime only arc written under supervision of the original author. That has been done in animated media before, you know.
Yep, because you can just go employing other companies workers. Obviously you can't do that!
An agreement would only be made if there were some gain to the manga publisher, and little loss. Explain me how advancing the story in the anime and not in the manga(or only way later in the manga), would be advantageous to the manga publisher.
A filler arc is an arc that will never be written in the manga and after it the story will return to the source material content(if not, the anime will just end with an original ending).
Also you're forgetting that while the anime is doing a filler, the author is continuing the source material(Money for the anime company, Money for the manga company). In this case you're saying for Togashi working for the anime and not for the manga(Money for the anime company, No Money for the manga company). So there is a big difference between that and a filler.