A carburetor is a balancing act...and pretty amazing for what it does, all mechanically. To go straight to your questions...
The float sets the fuel level, so the balancing act has a starting point...a base line. As you saw with your pressure experiment, the float is very sensitive to the pressure coming in, so that makes a difference in fuel level. The goal in an SU, though, so to end up with the fuel about 1/16 to 1/8"" below the top of the jet with the pressure your have coming in. The level must be constant or you start chasing your tail in the balancing act.
So, now the fuel is sitting at the opening of the airway in the carb. The size of the jet (the opening) is the next thing that controls how much can be sucked into the engine. Naturally, the larger the jet opening, the more fuel will be sucked in...so the jet size is selected for a particular engine. But the problem now is, the fuel flow is only controlled by the suction, with no adjustment for engine speed or load. An engine running slow at wide open throttle will produce the same suction as an engine running fast with very little throttle. As you can imagine, the fuel required for those 2 extremes is vastly different!
That's where the needle comes in. The needle fits into the jet, and can move up and down to fine tune the fuel that can be sucked out of the jet. If the needle is down, then the fat portion of the needle blocks most of the fuel from sucking out. As it moves upward, the needle is thinner, so more fuel can pass through the jet. For a particular engine, you could even have an adjustment that is not linear, by having the some parts of the needle thin faster, or even flatten out for much finer adjustment. Pretty cool idea!
The vacuum chamber is the key to needle placement. The chamber balances between a spring and the weight of the piston trying to push the needle down, and vacuum pulling the needle up. Here are some obvious conditions the needle can see:
At idle there is little air flow to generate a vacuum, so the piston falls to the bottom and the needle is in the jet all the way. So little fuel can be sucked out of the jet...perfect for an idle condition.
Now you floor it. All of a sudden you have lots of suction...especially since the piston partially blocks the air flow when fully down. More fuel is sucked out, and the piston starts lifting from the suction to let more fuel through the needle. The cool thing is the piston raises the needle at just the right rate to control the increased fuel as the engine speed increases.
Now you coast at cruise speed. The vacuum is medium, so the piston floats up and down to a balanced position for the particular engine speed and throttle opening. The needle sits near the perfect spot for correct fuel flow for that condition.
Way lengthy. The bottom line is the jet is:
Float sets the base line for the balancing act.
Jet controls the coarse fuel supply for the class of engine, say, cubic inches and number of carbs used.
The needle fine tunes the fuel flow, for say, cam or compression differences.
The piston dome spring fine tunes that needle position for best cruise mixture.
The dashpot slows the piston for rapid acceleration enrichment.
Don't get me started on the idle mixture and cold enrichener circuits...