How are you heating the water with your present system? I'm going to assume that you're taking city water and heating it in a boiler and then circulating the water or using a heat exchanger to transfer the created heat to the water. When you turn on your tap water and release cold water, that water is usually not a tepid 55 degrees, it's colder, which means that your system has to input heat from, say 40 degrees, up to what is needed. Geothermal brings two advantages, 1.) the water is sent into the ground and picks up heat, bringing it up to 55 degrees, or whatever the temp is in your locale (the sun heats all the earth around you and you're extracting a little bit of this solar energy from a cubic footprint of ground) and 2.) the heat pump compressor creates additional heat more efficiently than heating direct by electricity and natural gas. In a nutshell, here are the two stark alternatives: 1.) Let's assume that this is your system. Take city water at 40 degrees, burn natural gas to heat it to 120 degrees, meaning you have to burn natural gas to create BTUs which are then transferred directly to the water. 2.) A geothermal system with a loop field, starts at, say 55 and then the Heat Pump uses electricity and refrigeration principles, to heat the water up to 120 degrees but does so 3-5x more efficiently than a direct electric heater would. You're getting free heat from the ground (55-40) and the 15 degree differential (or whatever it actually is) is exploited by the heat pump which uses the embodied heat within that water to boost its temperature even higher and then sends the cold water that the process produces back into the ground where it soaks up the solar energy that has been "planted" into the ground and the process starts all over again.

The water temperature will most likely be too high. It depends on what type of radiant installation you will do. Typically a radiant heat zone will involve a mixing valve to deliver a lower water temp to the radiant zone. The existing boiler is used so I'm not sure if that is what you mean by a separate system.

Make sure you plumb the open system correctly where incoming water flushes the system when there is a demand for domestic hot water - this avoids stagnation during non-use (and inherit risk of a health problem). Open system is more efficient - 25% loss with a heat exchanger. It's harder to balance a single pump system as described. Ideal flow for each zone is different, and then the flow changes whenever a different combination of zones are working. It's not that it won't work, just that it might not work as well as having a properly balanced circuit for each zone. The wear & tear issue is also a concern as mentioned. It's hard to believe that the savings on pumps is all that much after installation of all that PEX, sensor circuits, and other stuff. Have you done the heating calculations that show 100 deg will get you to desired room (floor) temperature efficiently? That's a pretty low water temp for radiant. I understand protecting the wood floors. You could use a floor sensor and put it in series with your thermostat so the system shuts off when the floor temp threshold is met (I should patent that!). It's a surer way and could result in a more efficient system. Many wood floor systems just use the floor sensors in lieu of thermostats - that's another way. Note that high water temp doesn't mean high wood temp - it's all in the control setup.

I don't see why you can't install your bathroom fixtures first. Speak with your contractors and get their take. I'm sure there's pros and cons to either way. It may even be better to install the fixtures first. I would suggest meeting together with all the involved trades at one time to discuss what you want done and the order of construction. This way any objections can be raised and everyone knows what's expected and what they have to do so the other trades can do their part as efficiently as possible. Everybody on the same page... IMO SR