I don't think you will get a straight answer on the cure time. 28 days is definitely safe. Depending on the mix and temps during cure, you may be able to heat it sooner. I would be comfortable with low water temps after a week.
The required temps will vary a bit depending on the installation. Lower temps will give you more even melting, too low and it won't work. I like 90-95 for starters, don't go higher than 130, yes you can damage the concrete.
The slab sensor location is critically important. Find a spot between the tubes and out in the middle. You want a slab melt temp in the middle to upper 30's. If you have good downside insulation, you shouldn't need to "idle" the slab before a storm unless you are doing an airport.
For residential applications, 100-125 Btu's per square foot is a good design guideline.
You could probably calculate the actual time needed to heat up the concrete based on it's weight. A 4" thick slab will weigh about 50 lbs per square foot. At a specific heat of .18, that would be 9 btu's per square foot per degree. If the slab is 20 degrees before the storm, it would take 135 BTU/foot to bring it up to a 35 degree melting temp. Theoretically that would be a little over an hour. I reality, it would take several hours because of the heat being given off to the air. If you have inadequate insulation and you are wasting energy melting the ground under the slab, all bets are off. A pound of frozen water takes 144 btu's to melt from solid 32 to liquid 32.
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.