And thank you, Gustavo, for this feedback.
I find your statement
With units of measurement ... any noun phrases where such units are followed by "of" + a singular noun will tend to be followed by a singular verb at all times
and the accompanying example (g) to be in complete agreement with my comment
with any quantity of weight, we are dealing with a mass that requires a singular verb
and example (b1). If anything, your explanation is more precise than mine.
You make an excellent point with your follow-up statement:
I think the possibility to use either singular or plural will more likely arise when the partitive meaning is conveyed by a noun that is concrete, not just a unit
and the ensuing examples. I see that I was oversimplifying things, and that it is valid to see the quantity as a mass (thus requiring a singular verb) even if the quantity is expressed in terms of severally countable entities rather than units of measurement.
So, when "grain" is specifically being used as a unit of weight, we must use the singular verb (as per (g)). But if the context makes it clear that it is being used in the particulate sense, or even allows for that possibility, either the singular or plural verb is possible, depending on whether we want to focus on the substance (as per (h)) or the unit (per (i)).
I'll even take that a step further and suggest that, in either instance, it is possible to elide the "of ... " phrase, even if the object of "of" is ostensibly the actual subject of the sentence. Eg:
h1: Ten (bars/grains/bottles) is too much.
i1: Ten (bars/grains/bottles) are too many.
I think that you and I will agree that, nearly always, "much" must agree with the singular verb, and "many" with the plural. The only exceptions I can think of are certain instances when the number is "one". For example, an alcoholic might say:
j: For me, one drink is too many, and a thousand are not enough.