I think "If there were your help" sounds wrong for reasons that are completely unrelated to the use of the past subjunctive/irrealis were.
A test: do you think "If there was your help, I would finish quickly" sounds any better? What about "There was your help, so I was able to finish quickly"? Both of these sentences sound bad to me.
I think the problem lies in the use of the "there is" construction with "your help".
The noun "help" by itself seems more acceptable; "If there were help, I would be able to finish quickly" sounds better to me, and I was able to find an example sentence with this kind of structure: "Yes, if there were help of a certain amount given, we would establish these homes"(Scotland. Departmental Committee on Habitual Offenders, Inebriates, etc - 1895). I wouldn't say "If there were help" is everyday language, but it seems possible.
So why does adding "your" make it sound worse? I think it's because possessive determiners in English usually imply a sense of definiteness. When I say something like "your help", it is understood to mean something like "the help from you", not "any/some help from you".
And it's not usual to have a definite noun phrase after existential "there is" or "there are"; we say things like "There is a book on the shelf" but usually not things like "There is the book on the shelf" or "There is your book on the shelp." (These might be possible if "there" is used as a demonstrative, to emphasize the location of the book, but that is a distinct use of the word "there"—see the answer to "There is the man." Is *there* an adverb or pronoun?. When "there" is used as a dummy pronoun, it goes after the verb in questions—"Is there a book on the shelf?"—while when "there" is used as a demonstrative, it can't go in this position: we would say "Is your book there(,) on the shelf?" not "*Is there your book on the shelf?")