First of all welcome to the forum. To answer your concern about the comfort issue, there are butt plugs that have a different shaped base. It is common for people to experience discomfort from the conventional round base of a butt plug such as the one you own. The discomfort of the base felt on the outside of one's rear end can happen when sitting or after several hours when the lube dries out. I have also been meaning to try a plug with a different shape to the base, which I believe might be more comfortable.
I found this one:
I also found this one (in the picture the butt plug is attached to another device, but I am sure the butt plug can be purchased on its own):
Their site (designs such as the one you describe owning are pictured, but this is also the maker of the one in the twitter link above):
Notice that the base is a narrower, more linear shape on the examples above, which would rest neatly between the cheeks and might be more comfortable than the wide, round base on most other plugs. Yet the different shaped base appears to still be large enough to remain anchored on the outside of one's body and prevent the plug from sliding in too deep. I still do not know if sitting would be any better with these different designs, however.
I am not affiliated with the above sites/manufacturers.
As for the spontaneous leaking while wearing a plug, that has never happened to me. Some men drip while they are aroused and some do not. It is just a natural thing that can happen in a situation such as wearing a butt plug. Of course, most women tend to automatically lubricate quite a bit while aroused, and while wearing a plug anally. So it is a common experience no matter your anatomy, and not something to be concerned about. Just enjoy it!