"You throw very many fish on the floor" - in UK English using 'onto' would be mannered and a little odd. I accept it makes it easier to differentiate between in + acc and in+ abl - but it isn't a 'good' translation to write 'onto' here, I think.

Seems a bit unhygienic, don't you think?

Instead of 'very many', one might use 'a lot (of)' in the translation. So, "You throw a lot of fish onto the floor", which is better English.

Why is "plurimos" translated as "VERY many"? Why not simply "many" (or "most")?

two sentence before, "onto" was acepted. there are only two differences between the two sentences. one of them is the subject, and thus the inflection of the verb. both of them should be accepted in that respect.

Perfidus/a. (Or Perfide/a?)

I think "very many fishes" should also be accepted here. https://www.grammarly.com/blog/fish-fishes/