Customize the controls exactly the way I want them

No...you can hit rewind story. It's R1 on PlayStation so I'm assuming it's RB on Xbox. I don't know about PC. It gives you the option to rewind story so that the events and outcomes are different. If you hit replay mission and select a mission, you can rewind the story back to there.

I don't know about changing the difficulty, but after many tried and failed attempts on Veteran difficulty, I think I found an easy strategy to beat FOB Spectre.

I start by calling all my infantry to the ComSat station. I then take control of the CLAW near the power transformer. I left the cooling tower unguarded. Controlling the CLAW, I just sit at the power transformer and take out the incoming infantry and drones. This strategy allows me to keep the CLAW alive, while my incoming reinforcements continue to guard the sat station. The location of the CLAW also allows me to provide some cover for the infantry at the ComSat station by shooting incoming drones.

Once the cooling tower and ComSat get taken down, I walk the claw into the the building for the final showdown. Once inside, I was able to use the CLAW to keep the enemy forces at bay until just over a minute remaining. At that point the enemy took out my CLAW. I finished the mission as infantry, running and gunning into the factory to blow up any EMPs that were placed.

This method worked out well for me, and allowed me to beat the level the first time I tried it this way. I could not break the 5 minute mark before I tried it this way.

Good luck!