Put your shirt on, then the belt and buckle it/latch it till it's sitting loose enough so that the shirt can be adjusted down in the front. Have someone hold the bottom front of the shirt facing you, you lean towards them as they pull down, then have them hold it while you lean back. This should bring the collar down. When the collar is comfortably far enough from your neck, have the person tighten your belt while keeping the shirt in place. The belt should hold the shirt down on your body so that the collar doesn't ride up when you set up under the bar. If it does, the belt is not tight enough or you need to use a different belt that will...one that won't interfere with your breathing and can allow you to take in a big breath to hold when benching. Most benchers bring their shirts down incrementally depending on how heavy the weight they're trying to bench. If the shirt gives a lot of support and won't allow one to touch, they bring the collar high (but not to the point of choking them out). As they go heavier and can touch, they bring the shirt down so that the narrowest point of the chest plate (the part that gives the most support) is lower to the point where they bring the bar down. This increases the amount of support.
The pinching from the sleeves is normal, but only if the pinching is occuring right behind the triceps. A good denim shirt that provides a lot of triceps support (helpful for lockouts) will "bite" or leave bruising marks on the back of the arms and sometimes upper lats near the armpits.
Remember this shirt is new, so a lot of "learning" is needed in the shirt to get the groove right. Even then it still doesn't mean the shirt fits you properly. This is where the biggest part of the learning curve is in shirted benching. Learn to develop a high pain threshold...shirted benching is not supposed to be "comfortable".
