The coordinator would mostly be a UX win so that people might actually use it, manually pasting QRs and strings out of band is potentially a privacy shit-show and cumbersome

I'd intentionally not want to use the same lib across platforms specifically for entropy, just to dodge any mines it may introduce upstream like MicroPython did :RIP: ... just as Bitcoin Core taps RDRAND / RDSEED and a bunch of environment data directly

ShockWallet uses a keyring so your identity key just holds encrypted copies of each of the source keys, mainly for privacy from the Pub where desired... I'd think similar for something like this, each group you're part of has its own signing key and that's encrypted and unlocked by say a passkey etc

Policy I think just needs a primitive endpoint for a POC since who knows what policies people might implement, if you and I are in a group maybe one of us needs to make a withdrawal every week at the same time to the same address and a bot approves specifically those signing request (here I go talking about policy engines again)

People who can get low level on each platform for starters, but also cross-functionally make sure the protocol and flows are sane.