The OP makes good points. This should be posted to other fora.
But, that is actually the correct Latin. If you’d said “fori,” I’d be leaving your bleached bones by the highway.
To sum up: Loanwords can sometimes be pluralized according to the rules of their language of origin (but make sure you get those rules correct). And they can always be pluralized according to the rules of their new language. Which sometimes results in a word having multiple valid plurals, which is just fine. So if you’re not sure what language a word comes from, or how its plural worked in that language, you can always play it safe by using the standard English rules.
But since mention was made of plural words being loaned as singulars: Would the OP care to weigh in on Eastern European-style potato foldover dumplings? Is the English word for one of those a “pierog” or a “pierogi”?
I’m gonna call it a perogum, from now on.
Yeah, I know. But it’s not correct English.
Sure it is. Forums may be preferred by most English dictionaries, but find me one that says that fora is actually incorrect.
Personally I prefer to use the English plural, but in most cases the plural in the original language is still correct. (I must confess to preferring millennia to millenniums, however, just due to euphony.) And as the article cited in the OP points out, there are a few foreign loanwords that are never pluralized according to English rules, like testes, crises, and pyschoses.
I would argue that English has more than one plural scheme, and -us to -i is a neologistic irregular plural formation that transcends source language. Viri or virii or whatever is improper Latin, but in a descriptive sense is only arguably improper English. The reference here is octopi which is, of course, improper Greek, but completely common and understood in English, and will not stop being a proper, common, understood English plural no matter how much people insist on “octopuses” or “octopodes”.
I think “viruses” should be preferred, particularly in more formal registers like newspaper articles, but I also admit that I avoid saying/typing it because saying “uses” isn’t super easy and sounds extremely clumsy (which is a parallel reason to why I think so many people prefer octopi to octopuses, exact same problem). I’d also say the proper Latin plural “vira” would be only debatably correct in English because English doesn’t have the rule that forms that and would have wide difficulty being understood without explanation. What it does have is an -us to -i rule which, while inconsistently applied, is widely understood. It may stem from butchered Latin based off pattern recognition from the like 5 grade school words that still use that plural in English, but the tortured origin doesn’t matter so much as the fact that descriptively it’s absolutely a thing people intuitively understand and reach for.
Because my mother’s family came from Slovakia, we have always called those “piroh” in the singular, and “pirohy” in the plural. Pirog/pirogi is strictly Polish. And they’re not actually dumplings-- you don’t put them in soup.
duplicate
Should’t that be duplici?
Duplicata. Haven’t we been paying attention? 
DPRK: Which one means “this one is a copy of something”?
This is one of the best OPs I have seen in my time here. Bravo and thanks.
In Latin, opera is the plural of opus. But I thought in Italian, opera is a singular noun whose plural is opere. So opera had already become a singular noun by the time it was acquired into English.
And for an example of a loanword that almost never uses English pluralization, I’ve never heard anyone refer to “bacteriums”. Though, in practice, “bacteria” in English is close to being treated as a mass noun, rather than as a plural.
I’ve never had the need to discuss more than one octopus at a time.
I’ve always been partial to “virodes”.
Obviously I am going to defer to GreenWyvern for the straight dope on this one, but “duplicatum” would be the neuter perfect passive participle of “duplico”, so in the plural you get “duplicata”.
Actually, in English, you can say “bacterias,” as shorthand for “types of bacteria,” or “strains of bacteria,” like you can say “waters” as shorthand for “bodies of water.” “Bacterium” is used to refer to a single cell. As soon as you have two cells, you have “bacteria.” Take that one up with the medical community.
I find it peculiar that words like “paparazzi” and “graffiti” have quite recently entered the English language as words that are both plural and singular with the [del]Latin[/del] Italian singulars generally ignored.
And yet people get all wound up about the “proper” Latin pluralization of words that have been in the English for much, much longer. To me, words like “novas” and “formulas” are just fine.
So “viruses” is English. The Lain equivalent is not relevant.