My question is, Once these words enter English, how obligated are we to follow the original language in pluralization - or, for that matter, any aspect of their use, including their original meaning? We agreed among ourselves centuries ago to communicate in a language not burdened with fourth declensions and such, so why are we occasionally fixated with these in Modern English?

Not at all. I just find it funny that somebody would say virii instead of the much simpler and regular viruses. Same with octopuses, instead of the Graeco-Latinate octopodes. Or the exquisite ignorami for ignoramuses. The later is actually not even a noun in Latin, but a 1st person plural verb.


Ceci n'est pas un seing.