for onboarding i suggest to keep the secret key not even mentioned until the user is really engaged. and maybe even fake screens for email and password but email and password just unlocks the secret key saved in the device. after user is engaged now remind her to back up the secret key because email and password is nothing, secret key is everything. now she is more confused but engaged and confused 😉 just an idea!
for battery and bandwidth do we have to skip the outbox model?