1) No. Although I have never been a binger or one to eat past fullness. I don't know how some do this at buffets, and I have some friends who can chow down on 2,000 fattening calories in a sitting. I love food, but once I am satisfied at a meal, it's over. However, I noticed during my permabulker-fat guy phase that the more I ate, the hungrier and more quickly I became hungry between meals. So yes, I think the body does adapt a bit to the gastric stretching and possible hormonal abuse of overeating.
Most non-behemoth men with ordinary activity levels (say, six to eight hours a week of exercise activity total), need about 3500 cals to grow. This is a generalization and it's really not that much food to get this. So I guess man up and eat what you must.
1.2) If you want good digestion, then eat plenty of veggies and eat tubers and rice for your starchy carbs and some fruits. If you consume adequate amounts of animal products, there is no need to supplement with B12. That supplement is for vegans.
1.3) Please refer to some of the god-knows-how-many nutrition articles on this site, hundreds or thousands of them. You already know some high calorie foods: nuts, seeds, avocados, coconuts, sparing amounts of full fat dairy.
I am assuming your T levels are in the range of 500 to 700. If 500 or over with E2 of less than 50, you should feel alright. Besides, most docs would not give you T without being hypogonadal, say with a level of 400 something. My doc doesn't like seeing anyone with less than 400 with symptoms.
If one day you do become hypogonadal, you can bet your ass you're gonna want to "do the T thing"--TRT (medicine) to be exact.
2) Eat fish--simple as that.
No supplement or vitamin or mineral is going to raise your T levels if you do not have a nutritional deficiency, none. T raises T--end of story!
Vit. D is useful for those with little sunlight and vegetarians.
3) Plenty of people went from skinny to having a nice, muscular body, including myself. HOWEVER, if you have a diminutive frame, then there's no changing this. But ANYONE who has better genetics than Woody Allen can gain, depending on genes, 25 to 40 pounds of muscle over the course of five to seven years or so, sooner if they do things right and have great genes and little life stress and live like a monk.
4) If you limit cardio to low intensity 20 to 40 min a week, it can actually help your physique and health along the way. I myself run for 20 to 40 minutes three times a week at low intensity, or play games at the park in the summer, like paddleball, tennis, handball, etc.