SmartRover vs. in-dash.net HU - 2014 Mazda 6

So, I've been looking at pure Android HU for my car (a 2014 Mazda 6, Touring, with the Bose speakers) as an alternative to an Android Auto unit.

The Mazda forum I was checking out suggested one, which then became two more as the thread evolved. Two are by SmartRover, one by (sold by?) in-dash.net.

The two SR ones are Android 4.4, the in-dash.net is 5.1.

I don't have any experience with pure Android HUs, so a couple of generic questions:

1. These will probably never be upgraded past the version they're on, correct?

2. Is it possible to compile your own code to keep them updated? And if so, what accommodations would need to be made to not break the car functionality?

3. When making calls/using maps/etc., it's still pulling SOMETHING from the phone, albeit wirelessly. For phone, probably BT connection, for maps, either offline mode or Hotspot from the phone?

4. Will my steering wheel call button still active Google Now/some voice assistant on the phone, so I can activate calls wirelessly? (Hands free city. $500 fine.)

Here are links to the three units:

1. https://goo.gl/mJbCoH
2. https://goo.gl/51ItLo
3. https://goo.gl/MPJ8iz

As always, any and all help is appreciated.

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