If I don't withdraw all of my sats from my SN wallet (but also have an attached wallet), and I zap someone, does that zap only come from my attached wallet, or are the SN wallet sats depleted first?
We're still working on these final details, but they'd likely come from your attached wallet unless you've indicated you'd like to send Cowboy Credits instead.
And if it's the former, I guess a related question is, why would anyone keep sats in the SN wallet from that point forward?
They wouldn't choose to. But they might if:
  • they're new to SN and haven't had the chance to attach a wallet yet
  • their receiving wallet is unavailable temporarily
re: third party custodians
I can cope with a third party taking advantage of SN, but I will not stand for a third party taking advantage of any stacker by proxy. We will only partner with a third party custodian if I'm sure I can defend/save stackers from the third party should I need to.