On eBay you can get 10 diodes for $1.56 with free shipping from a US seller so I'd think you'd have them in a few days.
Or, you can get them 100 for $1 with free shipping from Asia if you can wait a couple weeks or so.
Anyway, take 4 diodes and connect them as shown. This drops the AC voltage by about 1.5V. So whether it's 18.9V as you measured or the actual nominal 20V, this simple circuit will drop it to below 19V. Each diode has a silver band on one end. The orientation is important.
For each 3-wire red-green LED pair, the upper and lower wires go to the AIU SW "1" and "2" terminals as before. But now the 6 center/common wires are tied together and go to the diode-dropper before connecting to the KW "U" (or AIU "IN"). All other turnout wiring is as before. This configuration can handle more (or less) than 6 turnouts should your needs change.
If you don't do eBay these are readily available components - something you'd find at Radio Shack if you can find one! 
