Yes, on the Marine Dress Blue "B" uniform.

Yes when appropriate for the duty-it's called modified dress blues

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Yes, an enlisted Marine Reserve can be buried in dress blues if it is the request of the deceased or their family. However, it is ultimately up to the family's preferences and any specific instructions or traditions they may have.

You don't. If you are not a Marine or a registered Junior Marine it is illegal to wear any form of military uniform. If you want to wear dress blues outside the privacy of your own home then you must enlist in the Marine Corps, otherwise you could be caught, convicted, and become a felon.