Glad to hear you are able to get it working, even if only with some difficulty.
Just to be clear though: the cursor does not change appearance when you hover directly over the splitters "triangles". But it sounds like you're not seeing the expected double-headed cursor for manual resizing (as you mocked up in your initial screenshot) when you locate your cursor anywhere on the splitter, correct? I also assume you are not using any special/custom cursor software, such as cursorFX.
Maybe take a look at the mouse settings in control panel (yes… control panel, not the more modern Windows10 general settings) and make sure the resizing cursor icon hasn't been changed somehow to be the same as the default cursor appearance. That seems highly unlikely, but if the cursor does not change to a double-headed arrow as you move along the splitter, that is the only thing I can think of that might cause it. I have never experienced a misbehaving cursor while using MC that you seem to be. Hope you can get it figured out.