"Reverse" frame is used for reversing the order of multiple frames in the Xsheet view (frame 2,3,4 -> 4,3,2). You need to select multiple frames for it to do anything.
Not sure what invert frame option you are talking about.
Are you trying the "flip" the frame to see what the mirror image looks like? (i.e -> <-) There isn't a convenient button for that, though in one of the forum posts, someone explained how to achieve this effect using multiple cameras. I tried it but you'd have to redo it every time you set up a new scene which doesn't make it convenient.
I think someone suggested a "flip" button in the GitHub forum but it's a feature request that no one has picked up to work on. :(