OK, I am amazed that you can get it to run with the tank that low. I think if you will move the tank up behind the engine, your problems will go away. The pressure nipple should be toward top of the muffler in any case. If you want to leave the tank where it is, you might rig an oil sump to fit in the muffler line. Maybe out of a film cannister or small pill bottle.

Looked at your picture and read the posts over again. Your muffler tap is OK. Generally we don't run that long an exposed muffler line. If your problem is only in cold, that suggests that maybe insulating the muffler line when it is cold might help.