Any place in Kathmandu, Pokhara, Jomson/Marpha or Manag where I could buy purified fuel for my MSR stove (Coleman, white gas). Alternatively i could bring a light gaz stove to screw upon gaz canisters with a screw but i guess liquid fuel gives me the best performance at high altitude/cold temperatures
I would like to trek from Marpha/Jomson to Dhaulagiri BC and back to Marpha. I guess i cannot take liquid fuel or gaz canisters in the plane flying Pokhara-Jomson
You are unlikely to be camping, where you will use the stove, particularly high up. I routinely use a gas stove above 5500m and have no problems with it. You need to buy the cans with an isobutane and propane mixture for high altitude use. They are available in KTM and Pokhara. I'm not sure about the other places but I suspect that you could find them in Jomson. I use the MSR WindPro stove. 80/20 mixes seem to work best for me.
I sometimes use a MSR WhisperLite International. It is a good deal more fiddly but will burn all sorts of things. In Nepal I use it with, well filtered, kerosene/paraffin that is available in towns/large villages. I have seen Coleman fuel in KTM but have never really looked for it. The gas stove is much more convenient after a hard day.
I use an 8oz/220ml can, or 20oz / 0.59l of liquid fuel, a week below the snow line. I do not cook anything involved. Think pot noodles for two. It all rather depends on how much cooking you are going to do, which obviously depends on what you eat.
You cannot take fuel on an aircraft.
Screw on gas canisters are sold by many shops in THamel.
Why don't you plan the Dhaulagiri Circuit clockwise like most people do (and I did) ? Indeed you will need to camp and to have a stove for this trek.
Screw on cartridges are widely available along the trekking routes, even though they are not needed unless camping independently. While standard gas mixtures do not work at cold temperatures at sea level, boiling point of propane-butane mix falls to around -25C at 5000m (around 0C at seal level), so higher altitudes help there.