I don't have SM, just a "regular" Pi4 running Kstars under ubuntu. 3A is fine for me, as my other equipment is powered from a separate 12v source.
Does SM need more than 3A?
I use one of these for my Pi4 in the field:
Another vote for a network share.
I share a directory I call NEW.CAPTURES from my desktop machine inside to /NEW.CAPTURES on the pi. Ekos saves all my FITS files in there, so they're all ready for PixInsight on the fast desktop machine.
Pretty easy to set up and very reliable.
To install NoMachine on your miniPC,
1) download the package from www.nomachine.com/download/linux&id=1
I don't know exactly what OS you've installed but I'll assume it's ubuntu on a 64-bit CPU
In that case you'd download "NoMachine for Linux DEB amd64"
2) Put the DEB file on your miniPC.
cd to that directory and type "dpkg -I nomachine_7.6.2_4_amd64.deb"
3) Install the software on the machine you want to use as a client (the one indoors, with a monitor)
Again I don't know what OS you use indoors, but you can install for Win, Mac, linux, iOS, or Android here:
4) Start the client program on your desktop doors. Click "Connection" and choose the miniPC
Let me know if you have trouble!
I use NoMachine instead. It's free, it has clients for Windows, Mac, linux, Pi, etc. And it works great.
No way can you power a 12v camera over USB.
Maybe you are asking if you can control the camera over USB if you power it with 12v DC? Then the answer is yes, but it's not power being transmitted by the USB. The USB port is for signals, not power.
The dummy plug doesn't actually do anything except provide passive resistors where the video card expects them too be. It "pretends" to be a monitor.
You can buy plugs that pretend to be higher resolution than HD. I think this one pretends to be 4K.
For just remote controlling a computer that has no monitor, I don't see the point in trying to get beautiful graphics. What you want is to do your capture on there and then display your processed image on a machine indoors. That's where the resolution will matter, not on your VNC session to run mount and camera control.