Mix your post workout shake with something that has a high glycemic index. I like to use PowerAid but you could use Gatoraid if you dont mind the high salt content. You could also use pure dextrose, etc. Approx. 100 gms
As for your L-glutamine, take 10-20 grms a day spaced out on an empty stomach. Take it between your meals.
As far as eating before your workout, you will just have to experiment with what works for you. I eat 1/2 hr before and do not have any problems, but only have a yogurt or something with high carbs. I would stay away from pretzels because even though they are carbs, they contain white flour, which is not good for you. As far as not being able to digest food and build muscle at the same time: you dont build muscle when you work out, you tear it. The building takes place when you rest.
Take your creatine after your workout to replace the creatine in your muscles that you exhausted from weight lifting.
Good luck