The MS260 one is 3mm compared to the MS200t being 1.5mm - this is the measurement from both sides of the diaphragm.
The difference in set up is that the 3mm MS260 metering arm is set level with the diaphragm bowl bottom whereas the 1.5mm MS200 is set to be level with the carb body - the part the diaphragm sits on.
Your description of an L screw tune is completely wrong. With your saw set with the L screw at 1 1/4 turns and idle around 2500rpm, if you turn the screw in, you should get the saw rising possibly 250-500rpm just before the saw dies. If your saw is revving as you say, either the idle screw is far to far in or the saw has a big air leak.
One of my own test with saws is that if you can wind the L screw all the way in and the saw still runs, the saw has an air leak or the idle screw is wildly to far open.
I would suggest having the carb off and looking at the throttle valve then adjusting the idle screw so the valve is ever so slightly open - easy to see looking at it against a bright light. Set the H & L screws to 1 1/4 turns, refit and initially adjust the idle so the saw idles OK and then do the adjustment.
The tank and fuel spurting....., the fuel in the tank gets pressurised when the fuel gets warm, if the tank is fitted with a ONE WAY breather that only lets air in, the fuel will spurt on disconnecting the fuel line. Some tanks have TWO WAY breathers such as the fibre pills and coarse screw in tube types and these will be less likely to spurt when the fuel line is disconnected. It is however, worth checking your fuel line for blockage and checking the fuel filter is not blocked. I have seen the part alloy ones have metal oxide blocking and issues in manufacturing or just old age.
Goodluck