Trench digging…

Typically your inspection card will clearly show when you can cover the pipes in your trenches.
Once you get the OK for Shotcrete most trenches can be filled.
Take a picture of the inspection card and post, it’s hard to advise on this subject because all cities are different on their requirements and time lines.

I don’t know of any requirements that would not allow all the pipes in the same trench. Inspectors typically are looking for the depth of utility pipes. The trench located to the left is for the gas line and will share the same trench.

I rented an small compact excavator (2 ton) to dig my trenches. My soil was rockier that yours. Plus I had a number of tree root to get through too. Also makes it easier to make sure you dig deep enough. The challenge is finding places to pile the spoils. If you can dig wide enough trench to work in and try to keep the spoils from falling back into the trenches it helps when laying pipe and conduit. I dug about 250 - 300 ft of trenches in a day (i did not stack my pool line and utilities). Plan and mark where you need to dig and also plan where you're going to stack the dirt. A wider trench is also easier to get a compactor in too.