The word dozen is a collective noun, i.e., singular when we think of them as groups and plural when we think of the individuals acting within the whole. So we might say:
Talking about eggs: "A dozen is probably not enough."
Talking about a party with friends: "A dozen are coming over this afternoon."
So, a dozen roses would likely be considered singular (like a bouquet). We might say, "A dozen roses costs ten rupees," as per subject-verb agreement.
1) However, since we have six dozen, does the modification of the noun by a number change the subject to plural, resulting in cost? Or is it all moot because of the plural roses?
In American English, collective nouns tend to be singular while in British English they may be both. That said, I've seen the suggestion that cost be used, which would be straightforward to me if dozens (plural) were used. From an American perspective, I feel the subject should remain singular when modified by a number: "Three dozen is enough to feed an army," or "Two dozen pizzas is/are too much" (the be verb is interchangeable in the 2nd example).
2) Is sticking with singular in the case above purely my American usage side showing, or is it a shared phenomenon?
When modified by a number, the plural is not inflected, that is, has no -s added.
3) (Main Question) Does this have any bearing on whether the subject is treated as a plural? That is, since a plural is not inflected, could the subject remain singular, or is this irrelevant?
Note this is not a duplicate, since that question is vague and about to be closed and my suggested edit, which is the basis of this question, was rejected.