There is no way for an independent developer to get the manufacturer to sign your app. The manufacturer will only sign their own apps, so that approach won't work for you.
And for operator signing, you would need to get every network operator (carrier) to sign the app separately, as it is dependent on whose SIM card is on the phone. So, for every country where you wish to distribute your app, you have to get every local operator to agree to sign your app. Usually, operators only sign their own apps, or sometimes apps they preinstall on their phones. In other words, that approach is extremely unlike to to apply, either, I'm afraid.