What is the correct plural form of the word "forum"?

This lack of respect for the language of origin not a phenomenon unique to English. When a word is borrowed into one language from another, unexpected things can happen.

I would argue that, for many examples you've given in your question, the actual perception of a singular-plural relationship is messy in practice, and the application of the plural is inconsistent.

Data: Using data as a collective noun with singular agreement is more common than using it with plural agreement. More in another thread from this site.

Alumni: I have heard as many people also use alumni for the singular, or even alum, as I have heard use alumnus for the singular. I imagine my experience with this word is typical (at least in the US), though certainly not universal. In any case, it is messy.

Media: The words media and medium don't even seem to correspond in any meaningful way in actual English usage. The word media has forked off and become a different word entirely. The word media is clearly used as a collective singular noun, as shown in newer constructions like multimedia (not multimedium even though we don't say multistages, multicores, multicycles, multistories, etc.). You will find few people who will ever say "Mass Medium". We talk about someone having "media savvy" even though we wouldn't say "computers savvy" (even though they can work with more than one computer). This is because, in English, these sorts of constructions always use the singular noun, whether it is collective or not. The way that media is used is evidence of how the word is actually parsed, perceived, and used by English speakers.

Another example of how foreign language morphology often doesn't mesh well: people try to pluralize octopus and virus as octopi and viri/virii, respectively. Virus was a mass noun in Latin, where we got the word. The word octopus comes from Greek and would take the plural form octopodes in Greek.

My main point is this: there is only a weak, inconsistent application of this -us to -a or -us to -i to begin with. So forums (like statuses and others) is a word even though we also sometimes have this other rule. Our language seems to continually push us towards either dropping the foreign pluralization in some way or another, or reanalyzing the plural as another distinct word. So I see this confusion as the language trying to mash these words around to make them fit our language naturally.

If we hadn't become so darn literate and knowledgeable in the past few centuries, I imagine these plurals would have regularized by now :)

1/ The article on the plural of... humf... err... penis is enlightening (emphasis mine):

The English-style plural is usually acceptable and often preferred.
When you don't know what the Latin plural is and don't have a dictionary handy, you should choose the English-style plural rather than try to guess.

[...]

In English -ums is sometimes preferred.
Forums (or fora), gymnasiums (or gymnasia), podiums (or podia), but bacteria (not bacteriums), phyla (not phylums). Seers are mediums but radio and television are two media.
Data is the plural of datum in Latin and English. English also has the plural form datums, but only in the cartographic sense (meaning a reference point). In English, purists still rail against using data and media with a singular verb. These are instances of usages that began as mistakes but are now so common that they are arguably correct.
Another example of the same evolution is agenda. In Latin (and sometimes in English) it is the plural of agendum (meaning "a thing that needs to be done") but is now almost invariably treated as a singular in English (meaning "a list or set of things that need to be done"), with the correct English plural agendas.
Plural forms that take a further pluralization (correctly or incorrectly) are called "double plurals."
Other examples include the incorrect forms alumnis and bacterias and the correct operas. One caution--quorum is a genitive plural pronoun in Latin. The English plural is quorums, never quora.

2/ The talk about uncountable usage on Wiktionary put the usage of a plural form in perspective.

It's not particularly unusual for a word to change from countable to uncountable as it is borrowed across languages:
virus:

• in Latin was a word meaning "slime" and uncountable,
• but in English used for specific objects and thus countable.

Similarly, as datum changed meaning from "gift" to "information", the pressure may have pushed data into a collective form (just as we generally say this information and not these informations—foreign speakers don't always catch this)

Regarding the repeated question regarding why websites say 'Forums' (plural) when you believe they mean 'Forum' (singular) - I would disagree. They generally mean what they say - the plural. Most websites have multiple separate forums covering different topics. So the question is, are they correct in regarding each compartmentalised debating space as a separate forum?

I'd say that becomes a question of architecture than linguistics. If the conversation within each is separate and disconnected from the other then they are separate and multiple forums. If multiple debates go on in a single space where all can be concurrently viewed then is is a single forum.

In the original meaning of the word, multiple debates may occur in a single town-square (forum) but where a town has multiple such squares they would be forums (ok, fora), even if the same debate goes on in each.