To disable Screensaver, go to Settings/Theme/Screensaver Start Time/Disable Screensaver.
Additionally, look in your TV settings (not the Roku), and see if you have control over the Display/Video Ratio (or similar language) over the TV video output or individual HDMI ports.
There is a reason that the Roku is overriding your manual settings. If you have another TV, try setting the 720p setting on the Roku while connected to that TV and then return it to your problem TV. (Recently, I have seen some postings that some users are reporting similar behaviors with Vizio TVs, so certainly could be a firmware compatibility issue).
For what its worth, my main TV (although capable of 1080) is set to 720p because I like the picture better. Original reason was similar to what you posted (I had a limited data plan with a young ISP who finally grew and now on 5000gb plan which is plenty for both work and play). I really only switch to 1080 for hockey games. Both my older and new Roku devices, though, do stay on the settings that I put them.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102