1. The rattle you hear when rotating the carb is not the emulsion tube (I've read things on the internet that calls it that but it really isn't). It's a fuel metering rod inside the idle/intermediate fuel circuit. It's not removable and, if equiped, needs to move freely. When reinstalling the low speed mixture adjustment screw, make sure the carburetor is not upside down. If the carburetor is upside down that rod will drop into the mixture screw bore and if the screw is forced in it'll cause damage.
2. The welsh plugs are designed to be installed with the domed shape outward. They fit loosely into the body of the carb. Once set in place, with the carb clamped in a vice with aluminum protectors, you use a large flat punch and hammer to flatten the plug. This spreads the plug outward and seals it tightly into the carburetor body. After instalation, it's a good idea to seal it with fingernail polish (I use clear).
3. I don't have a preference other than the diaphragm carbs can operate at a more severe angle, so they'd work good on a mini with an angle mount engine without the carb leveling manifold.
4. I don't know what the differences are between a 2 or 4-stroke carb. One misconception I read a lot is that Tecumseh carbs are the same for the 3 through 7 horsepower engines. They are the same type (series 1) of carb and they look identical externally, but internally they aren't the same. Take a 3HP carb and a 7HP carb, open the choke and throttle butterflies and look through them with a light behind them. The venturi size can be very noticably different. As for your next question, this (venturi size) also applies to snowblower carbs vs. standard carbs. The jetting can also be different. This doesn't just apply to the replaceable or adjustable jets, the internal fuel circuitry is different too. If you remove the welsh plug on the side of the carb that covers the idle/intermediate fuel chamber, the size, location and amount of holes (transfer pasages) is different too.
5. As far as jetting, that's something you do for your elevation and climate, but an adjustable carb makes that easy. I've never heard of snowblower carbs being better, they are jetted richer though. Something to remember with a snowblower carb is that they don't have the felt seals for the throttle and choke shafts so they're unprotected from dirt and grit entering and causing increased wear and air leaks. As far as the choke is concerned, the differences you've seen are probably just the positioning of the choke plate. If they get rotated slightly when tightening the screw that holds them to the shaft, it affects their movement.
