nice that you're interested in my project. I've also planned some other MySensors devices like an easy-to-use Raspberry Pi Gateway or battery-powered temperature and windows sensors.
To your questions:
I have written instructions on how to solder the adapter cables for ISP and Serial programming. They can be found on my website (https://thdev.org/?Projects___misc___micro_JST). There is a link in the building instructions, too.
I've purchased the 8pin connectors and cables from AliExpress. Here are the links to the offers. I have bad experience putting these in the BOM since AliExpress offers often expire quickly but these are still online.
If these should expire, just search for "jst 1.25", "smd jst 1.25mm" or similar.
Hope that I could answer your questions