Propter horum seditiosam atque improbam orationem multitudo frumentum non conferunt quod debent.
" On account of these people by seditious and disloyal argument the common people are not gathering the grain which they should." Assuming my translation is correct a single gender subject with plural verb (multitudo/conferunt in the case) always cause me problems because my first thought is a typo. I know the reasons why a plural verb might be used with a singular subject but how would the intent of the sentence be changed if the subject were plural?
Unfortunately you are wrong on your assumption. What case goes after propter?
Gender is masculine, feminine, or neuter, not single.
Finally, I don’t understand your question, but maybe others will.
See AG 317.d.2. It looks like Caesar uses a plural verb with multitudo on occasion. The example they give is B.G.ii.5
cum tanta multitudo lapides conicerent.
As for your translation I would change it to: “On account of the seditious and wicked speech of those (men) the common people are not gathering the grain which they should.”