Can I add 50/50 to plain water

No, you really shouldn't do that. You aren't really protecting anything by doing it. You're just putting a weak form of antifreeze into your cooling system, which will drop the freeze point a few degrees, but won't stop it from freezing. 50/50 coolant is ready to use. Don't add water to it.

If it does have only water in the system, there should be no problem running it out of the petcock and calling it a day. There'd be no reason to catch it as it shouldn't be harsh to the environment. Once you've drained about 1/2 of the water out of the system, then add full strength coolant to it.

Since you are in a winter climate right now, you need to get this done as quickly as possible or risk the water freezing in the block, which will damage it quite possibly to the extent of it needing a new engine. Don't hesitate to get this done.

I have the same kind of mind back in 2019. But after I did research online and discuss it with a mechanic, here it is.

1. Many product antifreezes have anti-rust or rust protection.
2. Car with plain water (tap water) will corrode the engine overtime because of impurities or minerals inside.
3. Adding plain water or tap water to Antifreeze defeats the purpose of antifreeze itself.

If you replace a car that decades using tap water to antifreeze you may harm your car than saving it. Remember antifreeze has some kind of anti rust or rust protection agent that removing your internal rust and if the rust or mineral deposit in your car is too much, ultimately the engine block or cylinder head could be broken with a hole.

if you replace with antifreeze in your car that only using tap water in less than 5yrs, you can and it will do good for your car.

that's why my 1991 car still using tap water until this day.. I don't want my block to have a hole because anti-rust in antifreeze coolant and I do live in a tropical country

PS: please don't just downvote without comment or explaining anything, if you don't have anything to says don't downvote, and you can literally crosscheck to google.

Yes of course you can add antifreeze or coolant to plain water. The question is why are you running plain water in an automobile?

If you are not comfortable get someone to add it. It is easy, and whoever changed your thermostat should have done this. If they won't make it right then have another mechanic do it. The cost of a gallon of coolant and an hour's labor are well worth it.

In addition to preventing freezing, coolant also prevents boiling which causes air bubbles and overheating. Coolant also contains additives to prevent corrosion (rust).