Essentially, you only want soup seasoned, so keep all thickening agents out of the soup. If you must use stock with thickening agents in it, then make sure you have approximately a third more water to start with. Top up with more just-boiled / hot water as you go (not cold or room temperature).
As far as flame / heat control goes, do what you did at the shabu-shabu restaurant: lower the heat when the water has boiled, increase the heat to boil or cook raw meat, etc.
And make sure wherever you're having hot pot that it's well-ventilated! You can easily turn your dining room / house into a sauna if you don't open the windows, or have some sort of extractor fan. Alternatively, consider a wind-protected area on the patio / in the garden. If you have an awning, rain / snow won't be a problem, but obviously, being outside depends on the wind chill temperature.