your devices can connect to your home hub BTWIFI but you should normally have your devices set to select the home network in preference to BTWIFI as BTWIFI is not secure
You need to ask your mobile manufacturer that. I've never heard of a mobile where you can't select wifi source.
If you ask it to Forget BTwifi and any other it selects then your home network should become the preferred one.
I see on Google Play the My BT app has only 3.2 stars. Is it that bad? I have not downloaded it yet. I would like to know if it will allow me to go roaming internet w/o further charges. Is that the case? I have BT Hub (latest 4?). Thanks.
The MyBT app has nothing to do with roaming, it is just for managing your account.
Your BT WiFi app that you already have allows you to connect to hotspots.