Yeah with a Schengen visa you can travel all EU countries (except Cyprus) Switzerland, Norway and Island are also Schengen countries. For UK you need to apply for separate visa.
just that there is where you get sorted re entry. There are multiple check points for immigration (4 i think) so if you go trough in the end of airport, you have to walk back to get re entry sorted. If so just make sure you have some spare time.
Just go and get it sorted. If question about your TM 30 just explain the issue. Probably they have you registered on computer, so no fuzz. Worst case scenario is a 2000 Baht fine
It's not that difficult to get a retirement visa/extention. It is actually the easiest visa to obtain without alot of hassle. All you need is money in the bank (800K) or confirmation of 65.000 Baht monthly income. A leasing contract for your recidence, and your passport. Done in less then a day
Just be prepared if things go sour, you will not be able to demand anything. I know many guys who has spendt millions renovating or building houses in Isaan, and when the marriage /relationship is over, they do not have any rights. So spend what you can afford to potentially loose, without you becoming broke is things turn bad