rbrt, I have over 40 years of experience with aroids but limited with tuberous types. So this is just my 2 cents. If someone with tuberous experience chimes in, follow their advice.

Definitely don't trash the tuber, in spite of your attempts to stop the rot, that growing point is surrounded by good tissue. I would try again to scrape or cut the rotted parts back to good tissue, if possible don't leave nooks and crannies with dark, rotted material in the creases. A powdered rooting hormone usually includes a fungicide, which might help. I've read that cinnamon has anti-rot qualities but I've never used it.

The great aglaonema expert Dr. B. Frank Brown recommends soaking seeds in a solution of 1 part household bleach and 9 parts water for 10 minutes to sterilize. You might try soaking the tuber, keeping the growing part in the center above the solution. I've done this with rotting stem cuttings of aglaonema with mixed success.

I would initially plant in a mix of 1 part peat moss or product like Miracle-Gro, and 1 part perlite, This is relatively sterile unlike peat mixes with bark or so-called potting soils with actual dirt. Pure perlite is good also, but looks the same dry or wet. I add a small amount of peat as an indicator of when to water.

Let us know how it goes.

Russ