> How is KDE doing with respect to QT, given that QT is commercial (with LGPL licensing) and has passed through several ownership changes?

KDE has the right to distribute Qt under a BSD-like licence after legal dispute.

> Is QT actively being maintained, and is KDE able to incorporate (or better - steer) those changes?

It is. KDE 6 is based on Qt 6.

> How are they doing with respect to the GTK/Gnome folks? (Did Gnome ever get over their issues? I tuned out around the time of Gnome 3 and the headaches everyone was having with Ubuntu vs. Gnome with respect to the desktop compositor.)

GNOME is still very stubborn but many of their works have come to fruition. KDE has adopted Flatpak and immutable OS.

> Should I choose Gnome or KDE for a desktop environment? (This is not a moral question! No religious fights. I'm seriously curious.)

Depends on your taste really. There are multiple rant articles about GNOME and I can write a fairly similar one about KDE. GNOME is the more polished out of the two, KDE has more features and has a less experimental workflow. Personally I also recommend trying out Pantheon, the DE of elementary OS.

Neither can reach the height of Windows and Mac OS X's prime since many UX issues are deeply ingrained, like FHS and XDG. You'll probably miss macOS application bundles.

> Which distro(s) have the best KDE? I've been stuck on Mac for a bit and want to dive in again soon.

Personally I like Fedora.