Oh... it's not "Me dijeron 'Gracias, adiós' " (i.e. "They told me 'thanks, bye' ") :-D
I find it interesting that in the goodbye, you put a person into God's hands in Spanish (adiós, a Dios) as well as in French (adieu, à Dieu) and in Swabian German which is influenced by French (ade). This makes saying bye to somebody a blessing somehow, you wish them protection and all the best.
Sure. The literal translation of the Spanish into English would be "They gave thanks to God." "God" in this case would be the indirect object, so it's necessary to use the indirect object pronoun "le" also.
Take a look at this article on the (redundant) indirect object pronoun: http://www.spanishdict.com/answers/100017/indirect-object-pronouns#.UL-nDY4goTs
Yep, that's right. "Le" and "Dios" are referring to the same person (the indirect object). There is no explicit reference to the subject here, but you can tell there's an implied "they/ellos" due to the conjugation of the verb.
- (Ellos) le dieron gracias a Dios. (They) gave thanks to God.
You'd use "les" in place of "le" if the indirect object were plural:
- They gave thanks to God. Le dieron gracias a Dios.
- The gave thanks to the gods. Les dieron gracias a los dioses.
"if the indirect object were plural:"
Good use of subjunctive here (for those unfamiliar with the English subjunctive.)
On "le", here is a good quote from a university Spanish professor:
• "Spanish prefers to use the indirect object pronouns even when the indirect object is mentioned."