1. Within reason but I find it is a combo of what diameter and length as well as what filter flow the CO2 is working against.
2. Kinda /sorta? The Co2 injected nearer the top meets the incoming water. This tries to force the CO2 bubbles down while they try to float up. So the best is that they go round and round, up and down but at some point the bubbles get too small to go up against the current and then do pass on out into the tank. Ever thought about how hailstones are produced? Same thing in reverse! Hail tries to fall down and get pushed up until they get too large and fall out.
3. Not much of a bubble or cloud of gas at the top because there is a constant flow of water going down. One reason for adding the CO2 in the center of the water flow so that it meets the stronger flow of water rather than the slower. less steady, flow near the edge. Think of the water in a river where the flow along the bank is slower than the stream center.
I find using Eheim 2217 on 75 gallon tanks, a single 1 1/2" with 18" of pipe and then adding a few fittings gets me total 21" and it never gives me any fizz in the tank or noise. Seems about right size to me.
Side or adding it at a fitting inline would seem to be much the same. I drill the side just to avoid using more fittings. Each fitting will decrease the filter flow a bit due to the way fittings are built. A fitting that is 3/4 on the outside is way less on the inside where the water runs. Too many fittings and the flow goes down more than we might want.
No good build posts but a couple ideas? Keep flow up as much as possible by using minimum number of fittings. If it fits your stand, etc. a gradual curve in the tubing will slow flow quite a bit less than a 90 degree angle fitting. If you watch rivers, you can see water running somewhat like car traffic. Curves slow things less than right angle corners due to the "turbulence" caused in each case.
The way mine are built?
Tubing from the 2217 (not the 2075 that is seen) through adapters to cut it down to the right size to go into a threaded plug glued to a coupling. This is done so that the tube can be pulled into the center of the pipe without a fitting. An 18" X1 1/2" pipe with threaded tee at the bottom. I give up some flow to go with the 90 degree line out at the bottom. The tee has a flat plug at the bottom because they are flat rather than using a cap which is rounded and makes the reactor harder to stand up by itself. I search for the black plastic (ABS?) irrigation fitting at the bottom as I like the more secure grip it gives on the tube without using a metal seal like at the top. I lose some flow but I'm willing to go with that.
I find no need of the purple primer when the pipe is new and not used outside when cold or wet. The normal glue softens the PVC fine when it doesn't have a lot of oxidation built up on it. If it's old pipe, use primer. USE fine grit sandpaper on all joints before gluing them up. Makes them fit the socket better as well as glue better. Test fit before gluing and don't work near flames. Outside is nice to avoid headaches?
Just my spin and there are hundreds of others out and about!!