Thank you for your answers, most of which were very helpful.

Do you think the versions below, where I have reduced the relative clause, are grammatical?

a.Thank you for your answers, most of them helpful

b. Thank you for your answers, most helpful

I believe they are grammatical. However, I would avoid them, as b is awkward and c is ambiguous (and awkward).

If you still believe they are ungrammatical, then would you say these identically constructed reduced relative clauses are also ungrammatical (these examples are descriptive in nature, a style more suited to these constructions)?

Similar to sentence a above: A horde of men, most of them leather-clad mobsters, ran rampant throughout city.

Similar to sentence b above. There, in the midst of the chaos, was a row of houses, all about to be destroyed by the angry, drunken mob.

Thank you
B cannot be used to mean A. Where is C?

Your 'similar' sentences are fine.
You must have misunderstood me. I have created sentences that match the two variations of the original; I'll reitterate:

Original 1: Thank you for your answers, most of which were helpful.

Here are two ways to reduce the relative clause:

a. Thank you for your answers, most of them helpful
b. Thank you for your answers, most helpful

Original 2: A horde of men, most of whom were leather-clad mobsters, ran rampant throughout city.

This reduction matches a.

A horde of men, most of them leather-clad mobsters, ran rampant throughout city.

Original 3: There, in the midst of the chaos, was a row of houses, all of which were about to be destroyed by the angry, drunken mob

This reduction matches b.

There, in the midst of the chaos, was a row of houses, all about to be destroyed
by the angry, drunken mob

Now what I'm saying is that the reduced relative clauses that structurally match a and b respectively are clearly grammatical, so why are a and b incorrect?
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A is fine. And I still don't see C, which you mention in your post. 'Original 3' works because there is more context, eliminating any ambiguity.