Donald Bell

Love it! Would this board accommodate the parts necessary for solar recharging that 18650 (PowerBoost?) or would that have to live outside the board/enclosure?

Yep. Depending on the size of your collection, a decently sized SD card may be enough to get it done, and you can skip the hard drive altogether.

Not without opening up your Echo. Like many devices, the Echo's minijack output is wired to disconnect the speaker when a cable is inserted. I'm sure it's possible to bypass, but messy.