You can definitely fix those
You can definitely fix those issues with your Harmony.
To disable turning off devices when changing activity, go to the activity, select “Settings”, then select “Chang options”, change “Turn Off Unused Devices”.
For the activities where your volume is not controlling your receiver, you need to run the set up wizard again on those activities and you will get an option which asks what you want to use for volume control.
The Harmony 300 will work. I do have one that I purchased recently. It certainly doesn’t have the bells and whistles of the One, but it is decent for the price. It is limited to a single activity, “Watch TV.” The configuration also is browser-based and it is fairly straightforward since there is no complication with multiple activities. It should work for your situation if you are willing to give it a try.