1) IP creep in a MK10 is always an issue with either the seat or the piston edge. There is no other possible cause for it. You can inspect the piston edge for a nick or scratch with a magnifying glass. If it's badly scratched or you see any imperfections on the edge, you can restore it. RSingler has a nice how-to thread about doing that. Usually they're ok. Considering I can see the groove in your seat from the photo even though it's not a very clear close up, you definitely need a new seat. You should take that one out, turn it over, and see what is engraved on the back. It's either a +, I, or II. Try to replace it with a seat that has the same markings, unless you know that you want to either lower or raise the IP from where it was before.
2) Did you put a yoke regulator on a convertible valve without the plug installed? I bet that didn't go too well when you turned on the air. Or was it a DIN regulator installed correctly but just submerged while not pressurized?
3) I'm not sure why you are looking for a left hand 2nd stage. Nobody uses those for sidemount any more, maybe some people did at first, but now it's typically a 7ft hose on the right tank and a 24" hose with a fixed 90 degree elbow on the left tank, routed behind your head. But, you seem to want to experiment in your own way, so have at it. I have seen left hand 109s, but they are fairly rare and probably expensive. I was going to buy one from a friend around the time I started sidemount, and then an expert sidemount instructor told me exactly what I just told you about the hose routing.
4) Other than finding a left hand 109, your only SP options are pilot, Air1, or something new like the G260. And I don't think you'd have a lot of success with the pilot or air1. They are difficult to work on and finicky. I love my pilots but I do not use them in overhead situations, only recreational single tank diving.
5) I'm glad that the MK10 I sent you is working well. When the MK15 arrives here (it's listed as late) I'll let you know how it works out.
