You can also use tasker+AutoInput installed on the HU (I don't use MDroid so I don't know its capabilies) to tap the button for you. This is what I was doing until the wifi launcher started allowing me to use my ODB wifi hotspot. I am not rooted (stupid Verizon), so I had a task to simulate taps to start the server then send an ADB command to the HU to start HUR and tap the wifi button after the phone was connected to BT, plugged in and connected to the car's wifi.Just to expand on what Clarkie49 said, because this is getting tiresome.
Disclaimer: my head unit's display is currently busted and I haven't been able to use it since these beta updates have been released with "native" AA wireless, however I doubt my head unit is compatible with it.
Using the WiFi Launcher method, the WiFi hotspot should be broadcast from the car deck/head unit, to which your phone connects. The SSID of this hotspot should be set to HUR and password AndroidAutoConnect unless you change these in the WiFi Launcher app on your phone, in which case you're phone will look for and connect to whatever unique SSID you specify.
You may need to confirm via a prompt on your phone to stay connected to this WiFi, which has no internet connection, and use mobile data.
Your head unit will not be connected to the internet this way so apps installed on the head unit will not update, etc. Your phone is connected to the internet, it just sends the Android Auto feed to Headunit Reloaded.
I don't use this method because I want my head unit apps to be up-to-date and YouTube Music to download Smart Playlists, etc. So, in case HUR craps out and I've no reception or my phone battery dies, I can still use native apps on the head unit.
My head unit has a crappy Bluetooth module and Bluetooth tethering from the phone isn't reliable so I connect my head unit to my phone's WiFi hotspot, which can be named whatever you want.
To automate things, I use Macrodroid on the head unit to launch HUR when connected to this hotspot. I used to use HUR in Self Mode and it was set up to launch directly into Self Mode so I never had to touch anything when getting in the car. However, Self Mode was laggy which was unsafe while driving so I've switched to using AA from the phone, which is much smoother and more responsive.
My phone is rooted so I'm able to use Macrodroid to start the AA head unit server on the phone (may not be necessary with native AA wireless, I'm not sure) with the following shell script, triggered when the phone connects to the head unit's Bluetooth after switching on the car:
am start-foreground-service -W com.google.android.projection.gearhead/com.google.android.projection.gearhead.companion.DeveloperHeadUnitNetworkService
...and stop the server with this one, triggered when disconnected from the cars Bluetooth:
am stopservice -W com.google.android.projection.gearhead/com.google.android.projection.gearhead.companion.DeveloperHeadUnitNetworkService
Macrodroid is also used to enable/disable the phone's hotspot when connected to/disconnected from the car's Bluetooth.
On the head unit, once it connects to the phone's hotspot, I have Macrodroid launch HUR. All I need to do is tap the big WiFi button and HUR connects to my phone's Android Auto.
As for your head unit not handling calls over Bluetooth, I'm not really sure. Maybe WiFi Launcher is starting when the phone connects to the car's Bluetooth, as it should, but screwing things up for you when you try to connect over USB. If you use your phone's hotspot, you don't need WiFi Launcher. If you use WiFi Launcher, you use your head unit's hotspot. Neither should affect the Bluetooth connection to your car's Bluetooth audio receiver (bar some 2.4GHz interference between WiFi and Bluetooth)
Sorry for the long-winded reply; I was just getting frustrated seeing the same things said over and over again. Poor explanations plague this thread; hopefully mine isn't one of them!
Please pull me up on any errors I've made here.