I have both in use. The new Nest Mini sounds much better than the Google Home Mini. It is minimally thicker, and also has a "hole" on the back so that it can be hung up easily.
Yes, you can combine several of these devices into a speaker group in the Google Home application. You can also set up a stereo pair.
Another interesting solution for better sound might be Google Nest Audio, which is a bit more expensive but sounds much better.
Google Nest Audio (Google Assistant)
No, these are Google Speakers that work with Google Home and Google apps. They are not bad, but there is the better Google Nest Audio, which sounds even better.
Yes, you do not need to have hub and steer all from mini nest. Moreover I'm pretty sure you don't even need mini nest if for example you want to steer your devices using android smqrtphone
I will say that the google home mini is the old name for the google Nest mini. I have one of each but no difference except that the HP is a bit better on the Nest
The control of Hue lamps that are connected to a bridge works absolutely problem-free with Google Home. I cannot say whether it also works with Bluetooth Hue lamps.
Here are some commands:
"Turn on [name of the lamp]."
"Dim [name of lamp]."
"Increase the brightness of [name of luminaire]."
"Set the brightness of [name of luminaire] to 60 percent."
"Dim/increase [name of luminaire] by 30 per cent."
"Change the colour of [name of luminaire] to blue."
"Switch the lights in [name of room] on/off."
"Switch all lights on/off."
If by "power cable" you mean the power supply unit, i.e. "power cable with adapter": I would call Google Support. Theoretically, you should be able to use the device with a xyz-15 V power supply, but then you will lose any warranty claims with Google. Why didn't Google equip the device with a USB-C port and standardised power supply? You'll have to ask Google about that too ...