IIUC, Mailspring (Nylas?) inserted their own cloud layer in between your device and a cloud email provider, to store email and provide "value added" features; so it's not cloud-free and requires trusting one more entity.
Nylas did, yeah. Mailspring stores all email locally. Recently the requirement to sign in with a mailspring ID was removed entirely.
I think they removed this but I'm not 100% sure
The Mailspring ID was made optional at the end of March. You do still need it if you want to use Mailspring Pro features that require their server.
Interesting! Does it support CalDAV?
Nope. It does have a plug-in api, though.
For which there is almost nothing available for at present.