[Paid Software Announcement and Discussion] journey42:
Found the magic!

Thank you so much!

I was about to try this, but gave the beta literally one more try, and I got it!

I figured out the magical order of things...

DEVICES: I am using a T-Mo S9 (stock, unrooted) as the parent, and a T-Mo S7 (working but cracked screen, debloated ROM, but unrooted; literally nothing but forced updates, XDA Labs, and AAGateway added) as the child.

PROCESS:

1) Setup of parent (best practice; some of this is probably unnecessary, but gives you a squeaky clean place to start from):

a) if installed, uninstall AAGateway

b) If on, turn off hotspot

c) force stop Android Auto, clear cache, clear data, clear permissions

d) clear Google Play services cache

e) restart in recovery and clear system cache

f) Open Android Auto and walk through setup wizard, enabling all permissions, but not setting any auto-open devices

g) In Android Auto, go to About, and enable developer settings, then exit the app

h) Install AAGateway from XDA Labs and open it up

i) Select 'Master' and check the auto-start tethering option, then exit

j) Restart the phone

k) Open Android Auto, go to the developer settings, and Start Head Unit Server, then exit the app

2) Setup of child (start disconnected from USB and with car off):

a) Go to wifi settings and - if previously connected - forget the parent's hotspot network

b) Force stop AAGateway, clear cache, data, defaults, and permissions - or uninstall

c) Go to Google Play Services, clear cache

d) Restart

e) Reinstall AAGateway if necessary, and open the app

f) Accept any permissions you can. Use the back button to get past the annoying USB default warning

g) Click 'slave' and check the auto-toggle WiFi option, then exit

h) Restart

3) Initial startup (car off):

a) On the parent, turn on hotspot

b) Join the child to the parent hotspot (turn on WiFi if needed)

c) Turn off child WiFi

d) Plug the child into head unit USB

e) On parent, open AAGateway AND TAP 'START MANUALLY' <--the magic

f) With both the parent and child unlocked and at the home screen, turn on the car and (first time only, if it doesn't automatically) turn on child WiFi

g) Allow anything the child asks for, set AAGateway as the default app on the child, click yes/accept to anything the parent asks for

YOU NOW HAVE WIRELESS ANDROID AUTO RUNNING!

Do not disconnect anything before you leave your car. Not the USB from the child, not the child from the parent hotspot...nothing.

Leave it alone until you turn off the car and the head unit turns off!

4) Ongoing use:

a) leave the child in the car, plugged into the head unit, off, and with WiFi automatically off

b) When you leave the car, force-close AAGateway on the parent, and manually turn off hotspot

c) When you next come back to the car, before turning it on (important), enable hotspot, then open AAGateway and manually start

d) wait a few seconds (very few; just enough to ensure hotspot is running

e) Turn on car. It should absolutely connect automatically.

Again...Do not disconnect anything before you leave your car. Not the USB from the child, not the child from the parent hotspot...nothing.

Leave it alone until you turn off the car and the head unit turns off!

This is rock-solid stable for me.

Don't forget that, after a phone restart of the parent, you need to Start Head Unit Server in Android Auto. I have it as a (totally manual, in my head) startup routine for when I restart.

I'm working on an automate script to 'physically' enable/disable Hotspot/WiFi and start/kill AAGateway (and Start Manually) on the parent, but so far, it's being finicky. It's worth the small effort of manual kill and manual start, though, at least for me. Looking forward to the next beta/stable!

Thank you [MENTION=2481649]fezster[/MENTION] and thank you Emil!