AFAIK the easiest way to make this game mode is to create 4 mods, where each mod gives 1 specific factions invulnerability, e.g. you'd have a mod where seraphim gets invulnerability, and no changes to the other factions.
Then it's just as simple as making changes to each Seraphim unit blueprint to make them invulnerable, or to give a "regen aura" that makes buildings around them invulnerable, and to make them uncapturable.
You could make 4 mods like this, one for each faction. You would decide in advance which faction(s) are for the hunters and which factions are for the hunted. Then enable some of the 4 mods as needed.
Making units uncapturable: I think there's a simple option in the blueprints for this, you just need to turn it on
Making units invulnerable: give them 10M hp + 10M/s hp regen, I think that stops basically any kind of weapon. The unit DB says yolo does up to 1M damage.
The only thing this wouldn't prevent would be the ACU dying in a transport, including if the transport gets shot down or if it tries to drop the ACU onto a cliff edge or if it tries to drop the ACU onto a building under construction. Not sure how to prevent that.
Making buildings invulnerable when they are near enough to a certain building: you could give shield gens a "regen aura" that basically gives an extra 10M hp + 10M/s hp regen.
I'm not sure whether you would need to edit literally every blueprint file for the mod, to make this work, or if you could get it to work just by editing the blueprints for ACU, shield gens, and making a simple edit to make every single building of that faction uncapturable (or maybe you need to make a separate edit to every single building's blueprint, but that's hardly impossible)
Also, I think it would be pretty easy to add a visual effect to the ACU, shield gen, and all buildings that get the special regen aura, e.g. like a personal shield but it's green or purple, that way it's very easy to tell what is or is not invulnerable.
I can't tell if you want to modify the existing shield gen or if you want to make a special "invulnerability generator" that just looks a bit like a shield gen. It would be easy enough to add a new unit like that.
Also, you might want to try to make it so that any unit that is invulnerable stops being targeted by enemy forces. That way the hunted player(s) wouldn't waste shots on them.