Noch vor Kurzem sollte hier ...
Why is "Kurzem" written with a capital "K"? How can it function as a noun here, and can we use the same structure with other adjectives (e.g., "vor Langem")?
Edit: According to the dictionary there is also a vor kurzem with small "k". Does this mean that both versions mean the same and can be used interchangably?
You realized exactly why it has a majuscule K. It's because in this context, Kurzem is being used as an adjectival noun. This can be done with any (I think) adjective. You just inflect it according to number and function in the sentence
Meinen Bruder habe ich seit Langem nicht gesehen
Er ist ein Mitreisender / Das ist ein Mitreisender.
Both variants are used. Duden for example recommends capitalization, but allows both. (It's rule 72.4 if the link doesn't work.)
I'd say "vor Kurzem" is the more modern variant, since it was introduced with the "Rechtschreibreform".
Capitalization is possible when an adjective is declined,
or when you use it as a noun in some other way (in this case there's an additional proposition).
"Gut und Böse" would be an example without declination.
Yes, you can nominalize almost all of them. One exception I could think of would be "entzwei":
Das Entzweie? Nope.
Following from that you could say that all adjectives, which cannot be used attributively, also cannot be nominalized. Attributive use of "entzwei" would look like this:
Der entzweie Baum. Nope again.
I don't know about "vor Langem", I'd recommend
seit Langem
vor langer Zeit
depending on what you want to say.
