Winterize ZR 409 closed cooling

2 Questions about winterizing.
Still trying to get to know the boat better (one year in).

2014 SN 200.
ZR 409 closed cooling with heat exchanger, Catanium exhaust

I've run through the PCM service manual's procedure several times successfully.
I like to ski at least once every 2-3 weeks in the winter.

To make winterizing more efficient, I wondered if it would work to add a t-fitting just before the raw water strainer to which I could connect a hose and deliver the pink environmentally friendly coolant (gravity fed from 5 gallon jug, circulated by running the engine for a minute or so).

If I add the tee, I have a 90 degree shutoff inline on the hose from the water pick-up at the hull I can close (to prevent coolant from leaving via the bottom of the boat). Coolant would fill the 3 feet of hose between my t-fitting and the shut off, then circulate.

When I winterized yesterday, about a quart of water came out of the Catanium exhaust manifold drain plugs.

I'm worried that with that much water in the exhaust manifolds, it will be hard to know if I get enough coolant in them (and water out of them) to prevent freeze damage. I hear the catanium manifolds are very expensive. Boat is stored in and indoor storage unit. Temps get down to 15 at night sometimes here.

My Second Question(s):
My driver (creature comfort) heater has the shutoff valve closed in the engine compartment where water would leave the engine and go to the heater circuit.

1. This circuit is coolant if I have closed cooling, right?
I opened a hose clamp, and coolant the same color as my engine coolant reservoir came out. I also traced the hoses and it seems like they are tied to the closed cooling system, not the raw water system.
2. If I open it for Winter boating heat, should I close it again in the Spring?

Thank you,
Dave

Hard to believe the manual doesn't say what and where to drain all the water out. Those exhaust manifolds are quite expense and absolutely should be drained. Once you get all the water out you are essentially safe for freezing temperatures. Some folks sleep better at night using antifreeze.

Not sure if the plumbing differs but in the '04 ZR6 - The manifolds drain by the common central drain system bolt - remove that and you will only get a quart or so of water. Make sure you remove your water strainer bowl and your raw water intake will drain - a lot will back drain once that airlock is broken. Make sure RWP and intercooler are drained. Intercooler should be the third hose coming to the central system drain bolt down the center of the bottom of the engine.

The manual should say to drain first even if you want to backfill with antifreeze. You need to drain first else you are diluting the antifreeze and raising the freezing point.