MHL is a system for cramming an HDMI-like video signal onto a micro-USB connector. It was invented to permit mobile devices to directly connect to TVs.
MHL has one one TMDS data-pair (vs. the four pairs used by HDMI), but the data carried over the line is otherwise HDMI compatible.
Your TV’s MHL-compatible HDMI port means you can connect an MHL-equipped mobile device using a passive cable (USB micro-B to HDMI). If you didn’t have that port (or if you want to use one of the other three ports for an MHL mobile device), then you’d need an active adapter, which costs more than a cable.
It might. It does on my system (Sony TV and Onkyo receiver).
With my setup, I’ve found that HDMI-CEC works surprisingly well.
If I turn on the Apple TV with its remote, it turns on the TV and receiver and also selects the correct input on the TV (or receiver, if it’s connected there).
I see the same behavior when turning on my (Sony) 4K Blu-Ray player. Powering it on will turn on the receiver and TV, and select the correct inputs.
If I turn off the Apple TV with its remote, everything is turned off (if the receiver or TV has the Apple TV selected).
If I turn off the TV with its remote, it also turns off everything.
But I also know that older/cheaper setups don’t work this well.
In another setup at home (TCL Roku TV, Apple TV and Sony Blu-Ray player), the Apple TV will power-on/off the TV. The Blu-Ray player will power-on the TV, but won’t power it off. Powering off the TV, however will turn off the BD player. And CEC won’t change the TV’s volume (the Blu-Ray remote can’t configure it. The Apple TV remote has to transmit IR for this).
And in another room (LG TV, Apple TV, Sony Blu-Ray player), none of the devices will power-on the TV, but they will power it off. And none of the devices will change the TV’s volume level (both BD and Apple remotes need to do it via IR).