Publishers can revoke stolen keys (and often do), but a retail key of the same region doesn't magically become "unauthorized" just because it was resold. That can't be legally revoked.
If a game is purchased from an authorized distributor, a US example would be Image, then it's still authorized when a store is reselling it. Authorized does not mean "bought direct from publisher."
Unauthorized would mean stolen, illegitimately obtained or out-of-region.
GMG has a history of taking care of customers, including refunding with Arkham.
No one, including yourself, has shown evidence that GMG is buying keys through unauthorized channels. If it is sourcing from random individuals, then yes, it's acting as an unauthorized seller. But if it is buying from authorized distributors throughout the EU, then there is nothing unauthorized about it.
As for Black Ops 3, GMG never advertised any pre-order bonus as coming with the game. If someone bought from them and didn't get Nuketown, there is no basis for complaint. The pre-order bonus was exclusive to GAME in the UK, which means no other shop in the UK sold BO3 w/Nuketown (this includes Amazon and other retail shops). It was not included "everywhere else" as you claim.
Battlefront has a release date of Nov 19 in Europe. GMG is based in the UK. Technically, it's not out yet for GMG. If the company is sticking to authorized sales, then wouldn't you expect it to honor the European release date?
From Amazon.co.uk:
Valve is the only one known to have done so. Before Steam had regions, Valve got pissed at people who were buying retail copies from Thailand and using them in the US. Those were all revoked and Valve basically said "tough shit."
GMG is based in the UK. The European release date is the 19th. How is that "not on time" if the keys are delivered by then?
Contest rules in various countries are a legal minefield. That has more to do with local law than any "publisher rights" around keys.