Antifreeze is formulated with anti-corrosion additives. These are consumed (reacted) during normal use. Using old antifreeze in which these additives are exhausted means that you are prone to corrosion during cleaning. More so due to the heat which accelerates corrosion. Even more so due to dilution with water. If you choose to use old, used up, diluted antifreeze, be sure you are cleaning your old, used up, dilapidated engines.
In industry, antifreeze may be recycled by filtering out all the crud and impurities and then putting in a new charge of anti-corrosion additives. Hopefully, that is where your used antifreeze is going when you turn it in to be recycled.
Using clean antifreeze along with heat is the most effective, safe cleaning method out there. Antifreeze with a lot of dissolved carbon and junk in it will clean slowly. If you are having poor results, it may be due to methods and materials. There are many other ideas for cleaning engines, and guys will use what they will use.
If your engine has a worn out piston liner fit after you clean it, it had a worn out piston liner fit before you dropped it into the pot. (Unless you took some kind of abrasive or polish to the pieces.) If the engine still runs without slowing down during the flight due to varnish, then keep running it. I don't quite understand the panic over cleaning a tired engine though. If you can get a wheezing Fox started, it will varnish up again. Just make sure you using all castor and run it hard.
Dave