I doubt seriously that Spectrum would allow this. Their app is intended for a cable subscriber to use it as an alternative to a set-top box at the address where the service is registered. You can only log in to the Spectrum app at the address where you have your cable service, so Guest Mode would no...
My parents switched to the Spectrum TV app for Roku recently when Spectrum started charging to use their boxes. The app, to be kind, is problematic. Calling Spectrum for support almost always results in a conversation with someone who doesn't even know there is such a thing as a Spectrum TV app.
It is not going to be possible to hook up your sub. You will need a soundbar with a sub out connector or one that is packaged with a wireless sub. Or go with an A/V receiver.
All the communication between TV and receiver will go through HDMI 1, assuming that's the ARC port. The other HDMI ports on the Roku TV would be for input only, as I understand it. If you are connecting all your external devices through the receiver, all the other HDMI ports on the TV besides the AR...
Roku Express Plus is the only current model I know of with analog outputs. No, there is no charge for using a Roku to stream Netflix outside of Netflix fees.