OK, but “some” with a countable singular noun doesn’t normally mean “at least one”. It means exactly one unidentified instance. “Some guy called for you” can only mean one guy.
I still don’t think we’ve really nailed why “form” is singular in this case. It seems that neither the singular or plural versions convey precisely the intended meaning. The singular version implies that each adult reported just one form, while the plural takes on another meaning of “some”, i.e. several identified instances.
Maybe the usage is just convention, since neither form is entirely satisfactory.
No, it’s the same reason as above. A person is likely to have only one form of sleepwalking. However…
I think you’ve found the answer - the problem is not the word form, but the word “some.” It can mean “a group of things” but it can also, as in this case, modify a word to mean a single choice among a group of things. For instance:
As for the OP’s sentence, I am taking it to include the possibility that some people reported more than one form of sleepwalking. If you’re excluding that possibility, we may be arguing slightly different points.
This is the answer to Reza’s question. They could’ve used plural, but most people just have one problem, apparently–and they want to emphasize that.
However, as for this:
I’m not sure exactly what you mean by this, but not every grammatical pattern has a “logical justification.” Grammar is really just a system of conventions, and sometimes the conventions would seem contradictory. To say that all grammar has a grammatical explanation is kind of a tautology.
Not in that sentence. “Some guy called” means exactly one guy called. Otherwise it would say “some guys called.” The word “some” doesn’t change that.
Right, it’s possible to have multiple forms, but that’s unlikely, and still implied. If you want to get technical, it should say “some form, or more than one form…” But who wants to read that?