Did you remove & test the thermostat at any point ? Also it's possible to get the two smaller pipes on the wrong ports, which does affect the coolant flow.
nothing wrong with running it for a while with water & always getting ot properly warmed up so you can find any leaks. Then after sorting those, drain & replace with proper coolant.
The original horn connector could also be similar to injector connectors ? I'm a long way from my P38's at the moment (Gran Canaria), but will look when I get home.
As long as it will mix with what is in there without turning to jelly.
I'd just top up with distilled water until the problem is sorted then drain ad refill with any antifreeze approved for alloy engines which is almost all.
Anti freeze should...
My two 1998 GEMS have different exhaust system. Green on has the usual dual tail pipe. Blue one has single tail pipe & sounds throatier. The only real physical difference is the section after the centre silencer box. Both are as I bought them.
If the dash is not asking for the EKA code then I doubt it's that. As Brian said, needs a security re-learn, read the code in the EDC and write it to the BECM.