Most GoPros are fixed frame rate. Only some of the non-Black (white or silver) Hero 7 were variable. I do not know about older than Hero 7 except I also have a Session 5 that is not variable.
No, that is a very risky path to having inaccurate edits! One must be careful with proxy mode because it can mask problems like this that are not limited to variable frame rate. It is possible seeking is not accurate in some combinations of encoding + container format. I can not give much specific as I do not have a list of known bad combinations, but I strongly suspect an open GOP encoding could be problematic. Most HDV camcorders used that.