Finally got it finished.
The water tank compartment is now even more tightly packed because of the larger system drain and all the connection adapters I needed to install. Here is a picture of the compartment (before reinstalling the heater duct):
Here is a close-up of the system drain. I tied into the PEX remnant from the original supply line, but had to use 1/2" id flexible tubing between the pump and drain valve:
Here is a close-up of the tank drain. I simply cleaned out and tested the old valve and re-taped the MPT connections into the tank:
Thanks for everyone's help.
