AC Handler Keeps Leaking After Multiple Repairs (Geothermal unit)
Questions

Q: What would I call the line marked D when googling. Water intake line?

A: "Intake" kind of works (we all understand it), but other helpful internet-search terms may include supply, PCV, and radiator. 

Q: I feel the problem is due to new pressure in the system since the leak keeps moving, how do I diagnose if it is the pressure and what is causing the pressure?

A: A water pressure guage is how you determine water pressure and I think it will probably be useful (to know, at least generally, the pressure you have). I think that you will want to install a pressure reducing valve and possibly a water hammer arrester as well. 

Also looking for any other advise or tips to figuring this thing out.

The problem might be vibrations. If vibrations are present, use a braided connector (for pressure above 25 psi) or you could use a rubber (radiator) hose connection (generally for pressure under 25 psi). Also, secure the pipes with straps wherever possible.

High water pressure can also cause leaks. If this is primarily a pressure problem, then a pressure reducing valve, and/or possibly a water hammer arrestor should help. You may want to install more than one pressure gauge (before and after) the radiator.