Well, The plate exchanger as you have seen can pull off more heat than the coil. Now that being said, it's much easier to get your water to what you need it to be with hotter water, vs, heating your home. You can heat your home with 120 degree water for example your fan just runs longer. However you can't get your domestic water hot enough with 120 degree water. Of course these are only examples. But having it before ensures your water is always hot and it doesn't affect the heat in the home p, your fan just might run a minute longer or something if your using dhw at the time..
Let's also keep in mind ridge your running your stove much hotter than a lot of folks. In just came from a customers house who had his stove set on 125 lol. But anyways, the affects for you are much less at 185 vs someone at 150.
As far as not needing your tempering valve, I dont feel that has anything to do with your coil being first. The only time it would affect it was when the fan and dhw were both being used at the same time. If you were to develop a small leak in your ot water domestic lines somewhere and it let it trickle your dhw could still potentially go to 180, the small leak would allow the water to enter slowly enough to equalize with the temp of furnace water. All be it a rare case, it is possible...