I usually paint with Terpenoid only medium first, and then go to 3:1 terp. to oil ratio, and then 2:1. Sometimes when forced to paint indoors, I may switch to Liquin over a surface that is dry to the touch but not cured for weeks or months. Am I dangerously viiolating the fat over lean principle? Is the Liquin too lean and quick drying?
Don't get too obsessed over "fat over lean". Most people start with thinner paint because it's just an easy way to sketch with the brush and work in more heavier mixtures later once you're settled in on the drawing or composition. Paintings aren't falling apart everywhere because people violated the fat over lean wisdom. Ideally, yes, on a molecular level it makes good sense. The biggest problem is not fat "over lean" but "new over old". If oil paint is applied around the same time, the paints weld themselves together. An older painting gets a hard outer coating and new paint will stick but not like it would if the curing was equal.
I have used every medium known to man, and have used liquin as a medium on older paintings and everything was fine. I will not paint over a varnished painting (of mine) without wet sanding the surface a bit. There I do feel it needs some tooth because i can feel the paint floating on the varnish. Generally, paint and enjoy yourself and don't worry about it. I never had paint flaking off or anything close in 40 years and I even have some 60's art school paintings (oil over acrylic) and they are very solid. These "rules" or guidelines are good ones and based on the wisdom of those who study the chemistry of paints. But I guarantee you, for all practical purposes, don't worry about it at all.
One last thing: Terpenoid is a fancy name for odorless thinner.