Help with circulator / parts for indirect water heater loop

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San Francisco Bay Area
I'd like to validate the parts selection for a DHW heating loop that we're adding to the radiant system. The radiant system has worked well for the past dozen years. It uses a differential pressure bypass to maintain flow instead of primary secondary piping.

I'd prefer to minimize changes, so I'm planning to add a separate DHW loop and use DHW priority. Because only one circulator runs at a time, flow should be consistent during either DHW or radiant heating.

Existing radiant system:

Add new DHW heating loop by tapping (S) and (R).

The parts to work with Lochinvar KBN-150 (135k BTU output) boiler:

Water heater: Lochinvar Squire SIT040 (Low resistance + KBN-150 integration)
DHW circulator: Grundfos UPS 26-99FC
Flow check for radiant return (R): Webstone H-10754W
Thermostatic mixing valve (S): Caleffi 521516A

My calculations are:

DHW loop pressure loss is 14.6@10 gpm, 17.44@12gps, 20@13 gpm ft head
(SIT040 water heater = 2.6 / 3.44 / 4 ft head @ 10 / 12 / 13 gpm
KBN-150 boiler + 20’ 1" pipes + 4 elbows + 2 valves = 12 / 14 / 16 ft head @ 10 / 12 / 13 gpm)

My main questions are:

1. Does the Grundfos UPS 26-99FC seem like a reasonable choice? For the indirect Squire SIT040, that should yield 12 gpm which makes use of the 135k btu output of the boiler.

2. I want to ensure 180F water will never flow into the radiant floor system, so I'm thinking about adding a thermostatic mixer at (S). I know the boiler will automatically reduce temperature to 125F during radiant heating, so am I being paranoid?

Plus, the mixer will likely reduce efficency of the radiant loop by 5-8 ft head...
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