1. No, you cannot. (There is technically the authnSel extension
which could in theory do something like that, but it is not actually supported by any browser.)
Android leaves to the user to decide what kind of authentication method they prefer. If you as an RP have requirements on permissible authenticators (for example, if you work in a regulated industry such as finance), you'll need to verify the attestation statement after the fact and reject the registration if the attestation does not satisfy your policy.
2. Either the browser doesn't support the extension, or the authenticator doesn't (or both). I'm not sure how widely the uvm extension is supported by browsers, but I think Chrome on Android does support it, so it might be that your authenticator doesn't support the extension. Also note that uvm is an authenticator extension, so you may want to read the extension output from the authenticatorData instead since the authenticatorData is signed but getClientExtensionResults() is not.
Software Engineer | Yubico