Troubleshooting is a big part of the DIY deal, learning to do it can be hard but it's very rewarding. Take your time and browse the internet for methods to troubleshoot electronics.
A few general tips:
Before all else, try reflowing all the components with a hot air gun and plenty of flux, check for shorts, clean up the board nicely. If it still doesn't work, check if you've used the correct parts, check their datasheet if they differ from the one in the Mouser cart provided in the build documentation.
If you are stuck, you could try asking for help in the dedicated Antumbra MuffWiggler forum or in the many SDIY forums.
Part of the DIY thing is sourcing components for yourself.
Take some time to find a suitable alternative component, compare datasheets and make sure that they match in all the important aspects. (Under important aspects, you might have to dig deeper what is a really important property of a specific component)
Keeping up with component availability is not my priority, I try to change components in the Mouser carts if they are not sold anymore.
The general answer is no, but usually you can find some Antumbra modules in facebook trade groups, eBay or on ModularGrid. You might also find a brave builder in these groups to build one for you on request.
No, they are only sold at ModularAddict currently.
If you'd like to sell Antumbra PCB/panel sets, please contact me at antumbramodular [at] gmail [dot] com.
All the files that are considered ready for release are listed in the module page/build documentation. Manual, build guide, BOM, Mouser cart, schematics, firmware code are all planned to be released for all modules, if they are not there, it's just a matter of time.
Eagle files are not planned to be released as of now.