Reducing swimming pool pump output from 1.5 to 1 inch

Circulator pumps can be used in open loop systems (though I think these are typically aluminum or brass rather than cast iron since the latter will corrode in an open loop system).

There is no way to know if your particular pump will suffice because we don't know the head loss of the hoses or the heat exchanger. I'm assuming the 13 M^3/hr figure is pumping against no head. You will of course flow less the more back pressure involved. You also have not said what flow you actually need. If you know the model number of the Grundfos, it should be a simple matter to download the pump curve. That will tell you the flow at various head pressures.

You could measure the head loss of the heat exchanger by putting pressure gauges on the input and output of the exchanger. The difference is the head loss. Bear in mind, however, that the head loss will depend on flow rate.

Probably the easiest thing to do is just connect the pump as you envision and see if you get enough flow.

The housing , after viewing the photo is attached to the discharge 1 1/2 inch plastic pipe. Although there are threads in view, you can also see a ring of glue (?) just above the pipe threads. So if there is no nut that would tighten to the pipe threads most likely you will need to cut the pipe to remove it from the housing. If the hole in the housing isn't very large you could try plugging it ,too.