We live in Northern Kentucky and for the last 10 days the AC has been running non-stop (24 hours per day - we have a NEST that tracks actual data) with an AVERAGE household temperature of 78 (the AC actually doesn't cool down to the anticipated 72 degrees). 

I recently did 1/2 thick travertine in a small bathroom & regretted it as it did suffer hairline cracks regardless of the solid thick backerboard/cement base its just the nature of the stone & more likely the temperature extremes we had last winter.

I doubt if we have the hardness you recorded but yearly preventative maintenance of any system is imperative to longevity.Given the precarious attic location I would definitely review the obvious advantages of an on-demand system.I have also designed a simple fix for the 'cold water sandwiching' issue that plagues the on-demand systems with feed water temperature fluctuations common in colder climates. 

I don't think it's a substantial amount of heat (temperature) on the backsplash, nothing it can't handle anyways. 

It is a simple fix - just sand and refinish the floor and make sure temperature in regulated in the summer (can be a challenge while renting).  

Pics of both sides of the wall may help evaluate.For what it may be worth:Rising water typically is ground water or flooding.Condensation has to do with the difference of temperature on opposing sides of the wall along with humidity levels (layman's definition for sure...)Penetration has to do with leaks and differences in pressure on each side of the wall (i.e., strong winds)Multiple potential culprits.  

The temperature is sensed right at the baseboard.

What if you could measure air temperature every 10 seconds to minimize heating and cooling costs?

I am trying to figure out what an acceptable temperature for hot water to pour out of the faucet or shower. 

It was stuffed so full that the airflow was cut off and the fridge was almost room temperature.