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"добрЫ". And it's better to omit "мне".
Yes.

"Будьте добры" or "будьте так добры" are modern replacements for "будьте любезны".

1. "скажите"
"сказать" is possible here, but... I whould say, if you read such a phrase, you think by default that is has a little bit "stern" or "command" tone. Without knowing context or hearing...