The way your setup looks to me is that you are filtering the oil then passing it to the cooler in the front of the car. Correct or not?If you want to have the option of only cooling or only heating the oil depending on time of the year I would take the line going from the adapter plate to a solenoid and switch it at will. If it is hot out then cool the oil. Switch to the in front RX7 cooler. If cold then the switch is the other way and the oil passes to the FPHE. A tee from the outputs of both back to the adapter and you have it. I wouldn't get hung up on oil temp too much, put in a gauge and just switch it once in a while during the seasons or if you take it off road. The standard warmer/coolers on the VW's don't even give you that amount of control so think about that. What ever the temperature of the coolant is the temp of the oil. So on a hard slow churn you might hit 230 F and the oil would as well. Splitting the system would allow you to cool the oil more and thus act as a second radiator, although a small one.
I run the larger V6 Passat oil cooler on my AHU. It is double the size of the factory one but still works with the factory cooling hoses.The absolute highest oil temperature I ever see is under 125°C which is totally safe for any modern diesel synthetic (I run Rotella T6 5w40). It won't hit that unless I'm climbing a mountain grade at speed for an extended period of time.Ideally you want your oil temps above boiling or close to it under normal circumstances. That's what gets moisture out of the oil. The VW setup is nice since the oil usually warms faster than the coolant so it helps bring the engine up to temp faster, then the coolant stays below the oil temp keeping everything balanced. It works well provided your radiator is big enough to manage your coolant temps.Same engine with the factory size oil cooler in my Jetta (smaller and lighter) would get over 130°C. I didn't like how much the pressure dropped at those higher temperatures hence I went to the larger cooler.The high temp high shear testing they do for oils is run at 150°C btw - that's supposed to be within its operating range. Sent from my LG-H873 using Tapatalk