Is OR Are?

Hello.
I have some questions on the agreement of subjects and verbs.
Why is the omitted in front of brightest in "The best and brightest," and not omitted in "The bold and the beautiful"?
Is it because in the first case, the best and the brightest are closely related so that you can group them together?

If that is the case, how do you decide whether to use singular verb in the following sentence?
The dedication and commitment which ___ demonstrated here might be crucial to your success.

Under the beforementioned assumption, "The dedication and the commitment" (let's call this case A) means that the author intends clear distinction between them. And "The dedication and commitment" (case B) signifies that author views them as one.

However, isn't it true that native speakers usually write phrases like "The first and the last" as "The first and last" to make a sentence flow more smooth?
If so, "The dedication and the commitment" (case A) could be written as "The dedication and commitment" (let's call this case A') and you wouldn't be able to tell the author's intention, since case A' could mean either case A or case B, correct?

In both "The best and brightest" and "The bold and the beautiful," I think what follows them is a plural verb because both "the best" and "the brightest" here represent groups and whether you view the best and brightest as one, it's still a group. But in "The dedication and commitment," each word requires a singular verb, and it might be that the in front of commitment is omitted. Does this mean you can use either a singular or a plural verb with "The dedication and commitment"?

I thank you in advance for you help.