PlasmaC has been under active development over the past year (or two?), I'd say check it out.
I haven't used the Proma, but you can definitely use it with PlasmaC. The Mesa THCAD is very popular for THC and fully integrates with PlasmaC to let linuxCNC handle height control. It's relatively low cost too. Because it's fully integrated it's smart enough to avoid those dives you're talking about and temporarily disable THC on small radius/low speed cuts. I don't know if that's possible with the proma, does it have a disable pin/signal? There are dozens of more knowledgeable folks on the forum that can tell you all about it, hopefully someone will see this and chime in.
Regarding the multipurpose table, you can just use a different linuxcnc config when milling or plotting. To setup the PlasmaC component all you need to do is get a basic working linuxcnc config, like for milling, then run that basic configuration through a tool called the configurator and it will add the necessary config files/settings to get plasmac setup for you, it's really not a huge mission. The PlasmaC user guide will walk you through it and there's tones of help here on the forum.
Do you feel that there's merit in my making further improvements available?
There's always room for improvement! But you may want to make sure you aren't reinventing the wheel.