It sure appears as though someone took off the relief valve and stuck on that mixer. That would be very dangerous.
It is possible that you have check valves installed in a manot that is allowing the tank to go over pressure with few other side effects. I am surprised if you are not getting water hammer or issues with appliance water valves.
If you can't find one of these somewhere in the system, piped to open air, you have a serious problem.
As long as you get the spacing correct, you field cut and sweat on a union when you trim it out. Generous pipe insulation will prevent contact with the concrete and give you a bit of wiggle room on the alignment.
For vehicle traffic, in my mind, the minimum compression strength should be 25psi. I like to think of the load path being a 1 to 1 ratio through the concrete, this accounts for poorly placed reinforcement (welded wire) and other variations that happen in the real world. The contact area of a heavy duty truck would likely be 4"x8", distributed through a marginally reinforce 4" slab, that would apply the pressure over a 12"x16" area of insulation. That would make the 2500 lb point loads required by code apply approx 13psi to the insulation.You then need to add the weight of the concrete (approx 1 psi) and a safety factor.
As for the r-value of Insultarp, I have used the product and been a victim of it's under performance. Our local building department no longer allows it because the manufacture cannot produce an ICC-ES report validating it's true R-value.
I post this one all the time but here it is again. Energy prices vary greatly by region. Fuels like propane and oil tend to be subject to more volatility that Nat gas and Electricity. Plug your local rates and guesses about future prices and make an informed decision.