@DiceKey 

> If I update my Deco M5's firmware to 1.6 without updating my M9 Decos, will I still be able to use the feature where I can select the signal source of my M5?

Yes, you can. That is what TP-Link says in documentation and what I tested with my Deco mesh (two M9s and two M5s).

> And client connection preference?

Nope. For it to work, all Deco nodes must be on 1.6 firmware. Even if it is enabled it might not work the way you assume it should.

> (Bonus Question: When will firmware 1.6 be available for the M9 Plus?)

When will 1.6.0 be released for the Deco M9 Plus

I am OK with partial bonus credit for that answer.

  @Alexandre. Thank you! Will update my M5 soon.

As for the thread regarding firmware 1.6 for M9 Plus, it's over a year old and there seems to be no recent update on this. Bummer.