If you're using a non-verb acronym or initialism as a verb, you're already in the realm of jargon. If you're writing in a context where that's acceptable, you should add a simple "ed" or "ing" for a suffix unless you're going for a humorous effect. When acronyms are absorbed into the language, they may acquire verb forms; for example, the verb meaning "to produce a laser beam" is "lase," retroactively treating the acronym for "light amplification by stimulated emission of radiation" as if it meant "something that 'lases.'"
I love your question. :-) (very unprofessional of me, I know.)
Acronyms are always capitalized, in order to alert the reader that the nonsense word they have just encountered is, indeed, an acronym.
I think the pertinent concept here is titles. Though common words appear in titles, the words in titles are capitalized.
When buildings are given a (common) title (as in your example, Student Union Building), they are capitalized on the building itself (often in the school's literature as well), because that is the building's title. You need not capitalized when you write about them using the common name. One would be surprised to see "student cafeteria" on a building, because that is the building's title, and as such, the words should begin with upper case letters.
Pertaining to example 2, your source may provide you a semblance of an answer in rules 9 and 10. "City Hall" is capitalized because it is a specific city hall, Vancouver's city hall. It is a title (Vancouver City Hall) for a specific building. Random or unspecified city halls or fire houses are not capitalized. Same with specific town halls (Trenton Town Hall), hospitals (Good Samaritan Hospital), etc. They are titles.
Pertaining to example 3, specific events, like books, have titles, and the first letter of the words in a title (with exceptions you are probably familiar with) are capitalized. So while people run marathons, they might not qualify to run in the Boston City Marathon; your dance is 'titled' "Winter Formal Dance". Wednesday Practice is also a specific, titled event, even if it occurs every week.
You might look at this page. It has a bit more detail on the use of capitals than your source.
Best Answer
The BBC is not as rigid about these matters as some other bodies. For example, it was recently written that the BBC had standardised on "CE" rather than "AD" for denoting years -- but in fact the BBC simply doesn't have a set of rigid standards for such things.
The Guardian Style Guide, however, states:
There is no universal rule; the Guardian's style guide is just one set of rules you can choose. The important thing is to be consistent.