Okay. Here is the heart of things as I see them. Shuki has actually talked about play being tiring. This is not the first time he has indicated that, at a certain point, his energy level and his ability to play at his optimum level drops.
As someone who can and has played for 12 hours straight, I could pretend to not understand that. But I do. And there are three parts to the solutions. All three of which need to be addressed together.
Shuki, you are a thin dude. You are healthy, but you may need more 1) strength and more 2) cardio endurance. But, that without the 3) nutritional thing (I know, I risk a torrential tirade of comical advice from Der_Echte by opening up this can of worms but I have a feeling that the tirade and his jumping on the bandwagon of condemning the pharmaceutical establishment that will undoubtedly go with it will be good high comedy--okay, I made myself laugh imagining it all) the fitness training without the nutritional thing will not be as useful. You need 1) Cardio Training, 2) Strength Training and 3) Good Nutrition.
Anyway, given how you are prone to ebbing energy levels, making sure you eat some good, healthy energy food an hour or two before you play would be really useful. Also having some fast energy food that has electrolyte replacements for you during play, would be useful. Like, every 45 min having half a banana! Those two pieces of nutritional advice alone, would really help your play time energy levels.
But, some strength training and a decent high-intensity cardio regimen to get you in better shape would be really useful so that you don't need to worry about how much energy you expend and the idea of trying to conserve energy can be overcome. Crystal Meth works too. But I don't recommend that route.
For the cardio, if you did something like circuit training, where you take a moderate intensity, and then push the intensity up for a period and then drop it down to the moderate intensity and then you spike the intensity and drop it. And each time you spike the intensity, you spike it higher till you hit max intensity which would be something like you running as fast as you possibly can for a minute and then going back to a level where it is still fast enough to be a workout but slow enough for you to "catch your breath", that kind of workout with those kinds of peaks and valleys in intensity done for 30-40 minutes straight, 1-3 times a week, will really help you get in better shape.
But you should probably also add a certain amount of strength training to your regimen: yes, lifting some weights. Those 2 things (done consistently will get you to stop worrying about getting tired while playing.
As far as heavy backspin and why it is something that is hard to return short, the speed of the spin makes it so that when you lightly touch the ball, the ball jumps off your racket much faster than slow backspin would cause. I know, it sounds obvious. If your racket is angled correctly to push the ball over the net onto the other side, the spin will cause the ball to go back fast enough so that it is much harder to keep the push soft. But, with that, we are talking about HEAVY backspin.
But that is the standard reason why short heavy backspin serves are so valuable. It is hard to loop them when they are short, over the table and really heavy. And it is hard to keep the push short so you make it much more likely that you get the first attack and you get a nice spinny push to attack so your loop should be pretty darn spinny too.
But, the kind of heavy backspin that really does that is someone like Damien Provost's heavy backspin. In other words, 2700 level heavy backspin. His serves are ridiculous to try and return.