I'm not sure you tried this stuff yet, but if I were you I would check out the Phoenix YouTube channel. They have videos that can help with everyone of these steps I would take.
1. Verify you do in fact have 7.1 installed, just cause it says it doesn't mean it is. Some of these Chinese manufacturers have been know to spoof the specs on the units.
2. Do the PX3 update, I've seen in many places that after the update parameter 7 should be selected.
3.Verify you have the right vehicle selected. There are many setups in our trucks and some have auto ac some don't, some have steering wheel controls some don't etc. This will make sure your ac settings are what they should be.
Also I think when these units were setup fir the 14-18 trucks they did it with the vehicles that have the radio controls on the steering wheel and the the color display in the gauge cluster in mind. Having these options is really helpful for making sure the right output is selected sometimes. If step 3 isn't done right the android tablet may not have some of the controls displayed or programmed properly. Like I said if you already did this stuff sorry wasting you time.
I just got google voice working today, so far so good. Fingers crossed it doesn't get slow. No I haven't checked out XDA Developers yet, I will. What did you search for to find the threads about these units. One thing I wish could be done is hide the icon bar at the top. With the Agama launcher I don't need it and on some apps it blocks the top part of the screen like Pandora where I can't access the top bar for searching and settings. Another thing that would be nice is like you mentioned having the NAV apps interrupt the actual radio like my phone does over Bluetooth. I was able to fix my wifi issue I think, thanks to you. Mine was not set to 48 hours it was the lowest one (can't remember what that is now though). I changed it and it seems to be functioning right now. If yours is soooo slow maybe try a factory reset and start over without installing the launchers you don't use.