I have assisted a friend of mine cleaning his aquarium and Koi pond. The style that you are using is mainly used for very large tanks or ponds. There is another T connector that fits onto many sink faucets and would be ideal to use for aquariums.
As far as the questions go: yes the main part (green pictured item) is a T connector with a 'shut off' for outgoing water. While yes, you can use it to refill, it isn't that great (it never shuts off completely). When I did his pond, we had the part that the water comes out in his garden area (did great in fertilizing and such) we refilled by hooking up a hose to a small 'tank' bucket that had a small carbon filter. We also used a small UV
light to treat the water as it ran through the tank (which warmed it slightly) and would then use a small hose to to refill the pond or aquarium slowly (not to change the water temp causing the fish to go into shock). Although we did not need to worry about the Ph of the water for the pond, the aquarium was a different matter. After the aquarium was almost full, we would check the Ph and add any declorination into the refill tank and slowly fill the tank the rest of the way. Checking again after about an hour or so.
Question 2 I cannot answer because I do not know the physics of the item, we never had a problem with any of the water back flowing...it did a really good job pumping out the junk from the pond (we used the big one on the pond and a smaller one that hooked to the kitchen sink for the aquarium) so I never questioned the how or why it didn't back flow...the pressure was enough to move some of the decoration rock at the bottom of the pond.
Question 3: Yes, but you will need to keep the water turned on if the elevation is too much. Again, I am not taught on the physics of how it works, but it does. If the elevation is too much, it looses pressure without the water turned on. The outside faucet for his Koi pond was elevated around 3 feet off the ground. We usually hooked a hose in between the connector and the faucet so we can direct the water better into the garden. His koi pond was around 4 feet deep and was in the ground. His aquarium was a tall 30 gallon (I believe) tank and was on normal height stand but this was all sitting in a low part of the house (the living room had 2 steps to get into the kitchen), the kitchen sink was typical height. We had to keep the water on low to keep the suction because of the elevation difference but nothing too major.
Question 4 I cannot answer because I have no clue as to what that is exactly so that means I have no experience with it.
Question 5, NO. I have seen the insides of a broken one and it is shaped different. The part that forms the base of the T (the part that actually forms the pump) has a slight curve in it that curves toward the out part. This may be the physics of how it works (to create the negative pressure). I have heard that it also is curved on the inside further down toward the hose part that forms the pump section (like a toilet bowl) but did not see this. I have tried using a regular T connector for another project (to pump out without electricity). It didn't work. I'm not understanding of the physics but I just know from experience. You may be able to create this if you have understanding on the physics of the unit and knowledge on how to put it together, but just a T will not work.
Question 6 was basically answered, we drained all water right outside into the garden, but there were times that we just drained right into the kitchen sink. To assist with splatter though, we used a collar/boot thing that held the shut off right down into the drain part itself (just wet it and twist/push into the primary drain of the sink). The further away the hose is, the more water is needed or the slower the draining will be (also the pump looses suction).
Last question..check the primer and glue to be sure it isn't toxic...most are not though. The solvent is glue and the primer just preps the pvc
for the glue. When the glue cures it will bond the two pvc
parts like one piece. The water will never touch it. Another Pastor friend of mine built a pond that was used for turtles, plants, and although it was designed to have Koi he just never got them in. He built his own filter unit and water fall and used PVC
pipe for the water. Most people do not realize that the modern homes use PVC
pipe from the main pipe to the meter and then from the meter to the house/building. I do know that there are some glues and primers that are not good though (my brother in law is a plumber and states that if it isn't good for landscaping then it won't be good for aquariums).
I may not have experience with keeping fish, but I do have experience in assisting in the cleaning of a 500 gallon Koi pond and a 30 gallon aquarium. I hoped this helped a bit. Maybe you can figure out the physics and show us non physics people how to make one for ourselves.