Since the TV remote is (mostly) working, you should be able to use it to get to HDMI2. To be honest, I've never heard of a case where adding some external device prevented a TV remote from working. Although sometimes HDMI-CEC can add some twists. I have turned HDMI-CEC off in my Samsung TV (Samsung calls it Anynet+) because I don't like "twists".
It sounds like the Roku remote may not be on an optimal code, given that the volume controls aren't working. (I think they're not from what you said.)
If you think the Roku is involved in this in some way, you could just unplug the HDMI port that it is plugged into. (However, I will be surprised if this changes anything – if it did, I would think some kind of HDMI-CEC confusion is going on.)
Just to add another point of confusion – A while ago I had to update the firmware in my parent's Samsung TV. According to internet chatter, some release of Samsung firmware was causing the TV to try to learn about the remote that was being used, but if you use multiple remotes, this caused its learning system to go haywire. In my parents' case, they lost all volume control. But since I updated their firmware a few months ago, all has been well. (I had to download the firmware onto a USB flash drive and plug it into the TV to update it.) I'm just grasping at straws on this one. Their volume control issue was pretty well-known on the internet. I don’t know about not turning on.
