> 1. Why is it so hard to keep track of recently recently touched files and folders in Finder?
This one's sort of on the user. Not sure how it could be simplified.
> 2. Why does the save/open dialogue constantly forget where I was just saving/opening stuff?
This is dependent on the application doing the saving, not on the Finder. Some applications are very good at remembering last-saved location. Photoshop and BBEdit are good examples of this, especially Photoshop which has per-menu-item preferences for things like Open, Save, and Export. However, many applications have developers not familiar with macOS conventions and so don't provide such affordances (I won't single out any applications because they really are too numerous to mention.)
> 3. Why doesn't opening a new tab open to your current location instead of ~/?
This is customizable in Finder > Preferences… I'm not sure what you mean by "current location". If you mean duplicate where you are now there are other ways to do it.
I personally have a AppleScript that copies my current POSIX path to my clipboard. I type Command T to open a new tab, Type Command-Shift G to "Go to Folder" and type Command V to paste what's on my clipboard.
But I'm not sure that's what you're looking for. You might head on over to http://discussions.apple.com and ask over there.
> 4. Why can't I completely customize the sidebar in a Finder window?
Yeah the customizability is restricted, mainly because letting it be too customizable means inexperienced users can get their sidebar where it's next to impossible to provide support. That said, advanced users usually know that the sidebar is only to store a few handy items. I mainly use it only for visual confirmation that my remote mounts and local disks are mounted.
I use keyboard shortcuts to navigate the file system in the Finder.
> 5. Why isn't there a native window switcher keyboard mapping?
I think you're looking for Command ` and Command-shift `. This works pretty much system wide (except for a few applications by software vendors that. Just. Refuse. Native. Keyboard shortcuts.)
> 6. Why do menubar items just vanish without a trace when menus need horizontal space?
Do you mean that your menubar is so crowded that you lose menu items? I think I recall this happening and, to be honest, I don't have any solutions here. You might head on over to http://discussions.apple.com. People can be really helpful there.
Good luck.