Is it possible to sign the application using a globally trusted certificate from like Verisign/Symantec or Thawte?
You can sign with whatever signing key you like, but it does not change the distribution options.
Is it possible to preload some kind of root certificate or public certificate on the devices beforehand and in this way make the device treat the apps I want to distribute to come from a known source?
With your own custom ROM mod and custom build of the Android OS, presumably. With stock devices, no.