There is a rectangle plate the came with the kit. You can use two layers of double sided tape to mount the fc.
I use the scotch brand grey stuff with the red backing. Its the best I have found as it resists water well and once the fc warms it up it holds for months.
When I tedid mine to upgrade to 4s I clipped the tabs off the outter edges since I stopped using the side plates. Then used 4 nylock nuts to mount it on the center screws so I was able to isolate it better as well as run wires under it better.
Theres some pics in my threads somewhere. The hard drive with those original pics gas died so I cant repost them for you.
ESC signal ground goes on the same ground as the ESC power. A ground is a ground. Good practice is to separate motor and RF/VT grounds if possible, as motor “noise” will disrupt the radio and video signal. The FC bec is there to smooth and clean up the power to the RX and video.