Okay, I have now solved all of the keyboard problems I was having, albeit that I had to set up things in different places. I will describe the solutions below.
1. The compose key problem
I managed to solve this by editing /etc/X11/xorg.conf.d/00-keyboard.conf. The file now looks as follows… 
# Read and parsed by systemd-localed. It's probably wise not to edit this file
# manually too freely.
Section "InputClass"
Identifier "system-keyboard"
MatchIsKeyboard "on"
Option "XkbLayout" "be"
Option "XkbModel" "pc105"
Option "XkbOptions" "compose:rwin"
EndSection
2. The keyboard repeat rate and delay, and the Mac-style “numbers-only” numeric keypad
I have tried the approach proposed by @omano via a script in Plasma’s Autostart section, but that didn’t work. I have solved it now by creating a new (root-owned and world-executable) script under /etc/X11/xinit/xinitrc.d/, called 70-keyboard_settings.sh, whose contents are the following…
#!/bin/sh
xset r rate 180 76
setxkbmap -option "numpad:mac"
The behavior is now again as desired, and as it used to be before whatever changes were applied by the update referenced in the opening post…:
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The right win-key now works as a compose key again.
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The numeric keypad now always produces numbers, regardless of whether Num Lock is on or off and regardless of whether the Shift key is pressed or not, just as it is on a Mac.
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Keyboard repeat rate and repeat delay are now as I want them, immediately upon logging into Plasma.
I am therefore going to mark this thread as solved, even though there may be people with yet other keyboard-related problems, to which I have linked here on this thread.
Thank you to everyone who responded and helped me on my way to finding the solution to my multiple and seemingly unrelated keyboard problems. 
