I can't find the study right now but you won't lose any extra fat by going low/no carb (if you're at the same calorie deficit). However, you will lose extra weight, but it will be water...
That's pretty normal from my experience. For T-shirts I wear a large or occasionally a medium, but for dress shirts I wear a large or XL depending on the brand and have the waist tailored.
Use sedentary for the days you don't exercise and use light/moderate for the days you do and average it over the week. You can either have a few days where you eat more because you're exercising...
That's not 100% true. If OP is a beginner lifter, it's possible for him to build a bit of muscle while eating at a deficit. That will stall out though.