Had a message from a friend earlier who is going home after just shy of 7 months (1 TR + Extension and 2 visa exemptions + 2 extensions). Apparently when the original IO scanned his passport they took him to a supervisor, the supervisor just scanned the passport again, stamped it and sent him on his way. When he asked what was wrong the supervisor just smiled and said sabay sabay (Left via DMK)
Sounds like this could be the most logical reason as to why
It will be announced like every other time a note/coin that has been removed from circulation. They will slowly remove old notes until there is only so many in circulation and then announce that you have until X date to exchange/use the old note/coin.
Non-O visa then Non-O extension to 1 year and getting a multiple re-entry permit is probably your best bet. If you time it right you'll be able to do it on your next entry.
The UK embassy doesn't care from my experience, I can't speak for the rest though. However your biggest problem will likely be with the airline as most will ask to see an onwards/return flight when checking-in
Yes you need to inform both the labour office and immigration which can be done prior to your last day at work.
Failing to inform immigration and the labour office can effect your ability to get another long-term visa and/or work permit in the future and will likely result in an overstay fine from immigration if you do not cancel your extension.