How do I install a keyboard layout?

First you should give the layout variant a name; see /usr/share/X11/xkb/symbols/se for examples. Then add it to /usr/share/X11/xkb/symbols/se and add a corresponding entry to /usr/share/X11/xkb/rules/evdev.xml.

Edit:

I tested the steps in the "update" section of your question, and it worked fine for me after having rebooted. The layout is shown in a submenu of Swedish (Sweden).

For testing subsequent changes, rebooting isn't necessary, but this command should suffice:

systemctl restart keyboard-setup

Edit II:

I have a theory (untested) on why the layout isn't working as robustly as you would wish. Try to change the two first lines in svorak-a5:

partial alphanumeric_keys
xkb_symbols "svorak" {

to

default partial alphanumeric_keys
xkb_symbols "basic" {