In my experience of hitting this issue today it's only an error if you've got your project set to upgrade warnings to errors. If you disable that you'll see NU1803 listed as a warning and not an error and the restore/build should work as normal. Obviously that's not really a soluti...
For 2. this could likely be solved by a proposedAllowUntrustedCertificatesorDisableCertificateValidationtype of flag. There will likely need to be different levers of control here. For example, one might be able to set both of these flags at both the global nuget.config level: ...