I’m no spring chicken and I cope! 
It helps if one tries to be consistent about which modifier combinations are used for specific contexts. For example (and simplifying a bit) I will often choose ⇧⌃ for actions specific to an application, ⇧⌃⌥⌘ for something that would work anywhere and ⇧⌥⌘ for activating a frequently used application.
I then keep a note of those combinations. Yes, I know most people won’t want that sort of admin, but it helps me to keep things in order.
That said, if a key combination does not quickly become something that one can use through “muscle memory”, then a key combination is probably not the best kind of trigger to use! We don’t have to use hotkeys for everything.
That’s a good example, perhaps. What proportion of actions in Word are used very frequently? For functionality that is needed no more than once per document, it’s probably going to be more practical to call up a list of options than to remember a key for something that is rarely used.
That’s provided you don’t need them very often. Please excuse me repeating myself on this point!
I don’t see how. For example, let’s pretend that one wants to use “Grow Font” constantly. What’s easiest to remember, [combo]+G or button 5? Which is easy to reach, won’t need you to look and won’t cause any significant interruption to your typing?
But what if all modifier combinations are already in use for G (that’s 15 permutations) or if it is just more convenient to use another key to stand in for a modifier? That’s where, in QMK, one might use an easy workaround that involves layers, but I won’t burden your attention further by going into details now. 
The initial setting up of QMK for a keyboard is a geeky process but if one follows the instructions carefully, I think it’s mostly all unfamiliar rather than inherently difficult. There is a learning curve, but it’s not rocket science, and how complicated it can become depends upon how ambitious one’s aims are—much as with Keyboard Maestro!
All QMK-programmable keyboards are compatible with Mac, Windows and Linux.
Tenting is built into a number of split keyboards, including the ZSA Ergodox EZ and the Dygma Raise. Some models, such as the Keychron Q11, offer optional tenting kits. For other keyboards, you can provide your own solution, from simply adding stick-on rubber feet to fancier DIY methods.
I’m puzzled that you should think so! Maybe it all just clicks with some people and not others, or maybe I was just especially motivated. Again I would draw a comparison with Keyboard Maestro; it’s beyond most people why anyone would want to bother with such an application, but those who have invested time and effort would agree that it pays off in the long term. 
Maybe that’s just my ignorance.
I think though that I have always made it clear that home row mods—not QMK—take a lot of tweaking of settings to get right, a lot of practice and a slightly different typing technique. They are far from being universally popular among QMK users!
Anyway, again, please don’t confuse such optional tricks with QMK as a whole.
