Case 1: Warning for John123 and reminder for CowboyPatroller. CowboyPatroller shouldn't have used a rollback in this case. He should have used "Undo" (giving a reason for undoing) or at least message John123. CowboyPatroller should be reminded for incorrect use of Rollback tool as well as failing to discuss the situation with John123, resulting in the user getting provoked. He may have genuinely thought this was spam/vandalism/speculation but there is no excuse for not communicating with the user about the removal of edits.
A warning for John123 would be justified. As this would be his first time that would not really deserve a ban. Calling a Patroller a "stupid prick" is uncalled for regardless whether the user in question was provoked. As with CowboyPatroller, he should have explained this in talk page, as well as showing some evidence of said musical instruments, and come to an agreement instead of continuing the edit war.
Case 2: I would talk to Newbie200 and explain to him how Wikia more or less works as well letting him know that he could ask questions in talk pages for the respective page. I do not believe he should be warned - mistakes like this do happen for new users.
GTAFAN316 would be reminded - even though he did the right thing by marking the article for deletion, that does not give him permissions to be harsh on users, especially new ones. "lrn2wiki" would not be appropriate, but it isn't a really bad offence so that doesn't need a warning - especially if the user had no history of bad behaviour before.
CowboyPatroller has tried to do the right thing by telling GTAFAN316 not to be mean but he should have talked to either users after that instead of disregarding everything. He also removed the delete template - I don't know if this was deliberate or a mistake - so I would ask him why did he undo the entire edit instead of removing the unacceptable comment. He might have been in a rush or on a mobile device where editing can be difficult so cases like this should be approached carefully.
Case 3: Since last block/warning was three months ago and the user in question has since improved by naming images correctly, I would explain to the user what to do and tell him to add license to images. If he still fails to follow the image policy, he would have to get warned and eventually blocked again.
Case 4: I would try to reason with the two bureaucrats. The staff team should co-operate and judging from the two bureaucrat's actions this is the complete opposite. I would ask others (preferably a bureaucrat) what they think of the situation and if they still fail to reason, I would contact Wikia staff and let them decide whether "TheWikiKid" is or is not underage. Hopefully the issues between the two bureaucrats have settled at that point, if not something would have to be done about this so I would most likely contact Wikia staff again.
Case 5a & 5b: I would try to reason with the user as both blocks are over a year old. I would explain to GTA92 that provocative comments, especially towards staff, would eventually result in him getting blocked for longer periods and explain that "Sky is Blue Trivia" isn't allowed. After he proceeded to remove my edits I would ask him for a clear reason as to why it is wrong. If the user fails to provide proper reasoning for the edit removals and admits to doing this out of spite, I would block him for two weeks - unless he talks to me way too aggressively then that would be a year long block or even a permanent one, depending on the severity.
Case 6: Aside from blocking the vandals, not much can be done about it. While it is true that GTAWiki and GrandTheftWiki were hostile towards each other, encouragement to join one and leave the other is neither a warnable or bannable offence. It's all down to personal opinion as to which wiki they like the most and to join it. If GTAmaniac would advertise on pages, noticeboards, main page, templates, etc around the wiki then that could warrant a warning or even a block, but there is nothing wrong with asking in talk pages. CowboyPatroller has no right in removing messages from talk pages regardless of wiki affiliation. I would not block GTAmaniac for encouraging to join the other wiki - only the vandals would get blocked since it's clear they do not plan on contributing.