How to absolutly prevent the engine from starting? (battery only emergency)

You need to call a tow truck. Most auto shop will eat the tow charge, if the work is done there; because it's going to be an expensive fix. An oil change isn't going to fix this. You've got coolant leaking into your engine oil - that means $$$$$.
Or you can rent a tow dolly and winch it onto that to take it to a mechanic.

 

Dreaded frozen white foam? Really?

It's pretty normal for gasoline engines to form an emulsion under the oil cap in winter even when everything is going normally. Repeated short trips without getting up to full running temperature is a great recipe for growing them.

And the next time the engine gets up to full running temp, the water portion naturally boils out and the remainder is closer to 100% oil.

Now, there are other reasons why you can grow mayo in there, and some of them are Really Terrible Problems that we probably don't want to think about until an actual mechanic makes a real diagnosis.

But so far it sounds like you might be overreacting to a natural phenomenon.

There is no starter to disconnect- the same motor-generators that provide your electric traction are the same units that start the engine. Lose one function, lose it all.

I would just get it started and drive it to a mechanic the normal way.

 

Besides, a PiP is not gonna get you more than 10 miles on EV only in winter even at (somethings very wrong ) creep home speeds. Are you within 10 miles of a shop?

If it were me I'd get an ODB2 adapter and a phone app CarScanner, DrPrius, Carista so you can look for trouble codes, that could give you and us at least a few clues of the extent of your sometimes shaky engine issue. And possibly save you a trip to the shop, even if that's an unlikely scenario.

 

How many miles on her?

if you’re not losing any coolant, just drive it to the mechanic.
If you are, it’s probably still okay to do so.
How many miles is it?

 

Either transport on a trailer-tow truck.
Or towing for a short distance at low speed with the transmission unlocked in neutral mode.
A well-charged 12V battery is required. Turn on the start button without pressing the brakes. Shift the gear selector to neutral.