I don't know if this is really an answer, but I've had a Coleman 6 person spa in service for about 25 years. Yes, 25 years! I have never had the same problems you are experiencing. We learned early on that any clothing (bathing suits etc) must be very clean and rinsed extremely well before going into the spa. Clothes cleaning detergents left in the fabrics release into the pool water and cause foam quickly. The clarifier is good at flocking these soaps, but they do tend to gum up the works quickly. We never lost a lot of pressure however, just had to clean the filter. We also made a habit of completely changing the water at least twice a year. After draining as much as we could via the drain hose, we used a wet vac to get as much out of any low spots. We then put in just enough water to cover the intake close to the bottom and jogged the pump to cycle clean water through the system and get the dirty water out. Again, drain and vacuum and repeat this process until the water looks clear. We do this with the filter out. This will clean out a lot of crap. About once a year just before we do a drain and complete cleaning, we treat the tub with anti-scale cleanser. This cleans the internal pipes of built up scale and solids. In your case, you may want to do this anti-scale treatment for a few days with the circulator on low and the filter out to purge the internal system. Then go ahead and change the water.
Another test you can do is to see how well the pump pushes water. When you have the small amount in the bottom, just above the intake, jets exposed, turn the jets to full, air off and see if the pump will shoot the water out of the jets with extreme force. Mine will shot water 20 feet away!!!! lol. If you don't have really good pressure, you may have a warn pump impeller. They can be damaged over time by chlorine based sanitizer, I never use chlorine, just bromine based shock etc.
The other item to check is if you have the right filter for your unit. Filters come in so many styles and densities. You may need to find one that allows better flow.
Assuming you find the root cause of your problem, good housekeeping, keeping as much contaminates as possible out of your tub will help. Very clean bathing suits or none at all,(yahoo) no body cosmetics such as body oils, moisturizers, perfumes etc. Keep chemical treatments to a minimum. Do a quick check weekly for PH, Total alkalinity, and bromine level. Adjust them sparingly so not to overdo any chemicals. Once you get everything under control, maintaining it is very easy and quick. Good Luck
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.
Best Answer
I think your pump head isn't enough to win both circuit head and circuit pressure. If the pump was designed to operate at this kind of load, then probably it has a burnt coil.