It may not be the carb. Just to be sure, get the saw hot, then set the L screw as rich as you can before the saw falters, then turn CW (lean) a small amount (say 1/16 to 1/8 turn). In this position, the saw is about as rich as it can be at Idle and there is no chance of having the progression from Idle to High too lean. The single screw carbs are a bit of a pain to diagnose.
Your points:
1) Maybe... but in general, if the welch plug is loose, you get an over rich condition, not a "lean" condition. If the transition ports (3) are blocked or partially blocked, the "falter" condition will tend to be permanent, not intermittent.
2) yes... you could have a slightly bad impulse hose that only fails occasinally due to a small crack.
3) yes...
2 and 3 are easy to check. Just take them off, screw in a 5mm screw (just borrow one off the saw) into one end, and pump up the other end with a bicycle pump or whatever to 10psi. Spray soapy water on the hose and see if it leaks.
Unless you're good with carbs, the easiest way to eliminate carb "problems" is just drop on another. I have a few brand new 026 dual screw carbs I can sell for low $. let me know by PM if you need one.
There is another possible cause related to air leaks. The oil seal under the flywheel fails, and at first fails intermittently. This affects the idle stability more then the aceelaration, but I've been both. You'll need to vacumm test the saw to figure out if this is failing. As it's tiny and often set too deep, it's a b???h to replace without the Stihl oil seal puller.