Did I make a big mistake with my stone foundation?

I built a foundation of stone and mortar for my 30ft yurt. I dug a couple feet into the ground and installed a french drain (4 inch perforated pipe surrounded by crushed stone all wrapped in drainage fabric, sloped to day light). I packed two inches of stone dust on this and started the rock wall.  The rock wall is 2ft wide at the bottom, 10 inches wide at the top. Ive never noticed any cracks or movement in the foundation, the yurt has sat on it for over 4 years.

My concern is, I live in Maine and my frost line is 4ft deep. I am nowhere near the frost line with my two foot deep foundation.

Another concern, I think I should have had the drain around the outside perimeter of the stone wall. Not directly under it all. I am worried the drain could someday cave in and cause a problem.

I have been slowly turning the yurt into a more permanent structure and adding weight to the walls. Am I investing more into a structure that is doomed to have structural issues, or will my wall hold up?