Hi
I have performed quite a few changes on my Wee with no issues, drain oil and change filter, what I do then is add 2.3 ltrs of oil, run engine for a few mins to fill filter, then let it rest a while and then top up to full line in sight glass, I thought that it was 2.7 ltrs with filter change, but Glee may be different from Wee.
Regarding froth, never been an issue, have noticed in winter a bit of misting in glass, and mayo on the filler plug. There is a small sponge in the air filter that requires cleaning out now and again, could cause the misting if really wet.
Yes, cooling fluid can get into the engine just like a car engine, via head gasket or other places, but you would see a drop in the cooling fluid if it was using it, and if it was getting into one of the cylinders, quite a lot of steam from exhaust.
I really do not know why your level is going up and down as it is, are you checking on the main stand ?