Three part question, but one answer should suffice.
Q:If Ichigo had been killed by Ginjou instead of him simply taking away his fullbring would Ichigo's spirit have returned with his normal shinigami attire and abilities to deliver some payback? Or due to an I believe still severed chain of fate cause hollow ichigo to emerge and kill Ginjou and Tsukishima(Possibly Ishida and co. too)? Or would he simply be a normal dead spirit?
Q: Even if Aizen had managed to create the Oken and go to the Spirit King dimension, would he be able to defeat the Royal Guard known also as "Squad 0"? The members of Royal guard are all above captain level I guess...so the strongest member of the guard must be a real beast! Would Aizen be able to kill them all and get to the soul king?
My answer is:Yes,98 percent sure that he could have trashed them without much struggle.The spirit king itself won't be active in the battle,only the squad zero's defenders are the problem.Their only mission is to protect the spirit king,meaning the spirit king can't fight.
If Aizen wasn't sealed by Urahara Kisuke,he would have evolved on entirely different level than Ichigo using Mugetsu.So that's why I believe that the winner,without a doubt in this theoretical battle is the Mind-Fucking Beast named Aizen.
He has the Hyogyouku,which makes him truly immortal,he is also one with his zanpaktou(the only other one who has achieved this is no other than the defeated Ichigo),his Kyouka Suigetsu is absolute hax and etc.. .
It's hard to say if they could have beaten Aizen if he reached them, we don't even know if Yamamoto could have beaten him. Or if Yamamoto with Kyoraku and Ukitake could have beaten him. These are all so awesome that we havn't even seen their Bankai and Yamamoto could trap Aizen, Tousen and Gin with his Shikai, tho Aizen probably could have broken out at full power.
Then imagine if Yamamoto and kyoraku n Ukitake didn't get promoted to squad zero could mean 2 things:
1.st : There wasn't any need for a new member in squad zero since Kirio Hikifune joined
2nd: They weren't strong enough to get promoted, which can give us the idea of their strenght.
Personally I think that they are stronger than Kyoraku and Ukitake, but maybe they weren't promoted because they simply refused..? o.o