Question 1) is a general one and hopefully got sufficient reply. I will try to answer the others:
Question 2 & 3) Yes, it is perfectly allowed to wear Jeans, T-Shirt and the like. Likewise, there is no problem in tucking your shirt into the pants, by default. The problem is when the pants are not loose enough to hide your 'awrah'. Now, the 'awrah' of the men essentially stretches from navel to the knees, but what is often overlooked by some people is the proper manner of concealing the 'awrah', which besides hiding these body parts with a cloth also include proper tailoring of the garment. That is, if the pants or trousers are so tight as to reveal what one should be concealing like the curves of thighs/buttocks or loose but sticking to the skin like an adhesive and thus outlining the inner structure. Eventually, some garments can be alright while you are standing but all revealing when you bend e.g. in Sujood. Therefore, all with the matter is relative to situations and a careful choice about one's dress is the way out.
Question 3) Hopefully, it is for the above reason, it is desirable to wear a long shirt reaching near to the knees like a 'kurta' or 'milder' styles like long-tail 'plumber' shirt. In the same line, avoiding tucked-in shirts gives some degree of satisfaction to others, which for example in the Indian Subcontinent is also taken as a sign of humility. Cultural tendencies always tend to cast their influence. Hopefully, it is for the above reasons, scholars in the west too embrace this idea of greater modesty and prefer long Jibba over short coat.
And as a matter of fact, in such cases where a general ruling is not possible, most of the people are left with misconceptions and exaggerated conclusions.
"I personally prefer a long shirt over a short or tucked-in, a Jeans
over other material, and a knee-length over all. Finding the right
pants almost always gives a tough time but it is worth it."