Let me see if I can make a few suggestions! I would like to preface this response with a note to make sure that any of these are appropriate for you personally by discussing them w/ your healthcare provider. Even though you've outlined some health history, I would need much more to make a personal recommendation.
So, one, for weight gain, I'd suggest adding in some higher calorie ingredients like nuts, seeds, nut butters, cold-pressed coconut oil, avocado and also some added protein to prevent further muscle wasting. Here's an article on protein powders: http://www.nutriliving.com/blog/which-protein-powder-best-you
Aim for at least 72g protein a day (possibly even up to 1-1.2g/kg body weight) to maintain any lean body mass.
You received some good suggestions from the other NB user...
Re the diarrhea: the main goal is to figure out what is disturbing your digestive system. If your previous diet was lacking high fiber foods and you are now NutriBlasting, this could be a sign that you took it too fast...cut back on portions and introduce fiber slowly. Choose bland foods like banana, applesauce, rice and try some peppermint or chamomile tea to reduce inflammation and soothe your GI. Iron supplements help dry up loose stools. Also, be sure to rehydrate. You lose a lot of water and electrolytes when diarrhea occurs. I'm not sure how this would taste, but how about banana, oats, apple, a dash of cinnamon, pumpkin seeds, and coconut water!
I would start by using your short cup and make sure you are drinking slowly. I know sometimes I can drink a Blast in under 5 minutes and this usually makes my stomach hurt. Try this Blasts and slowly build up to including more raw veggies.
1/2 banana, 1/4 oats, 1 cup almond milk or coconut water, 1 mint leaf, 1/4 inch chunk of ginger and 1/4 cup of cherries or kiwi or any fruit you have on hand. Drink this slowly and see how it sits.
For inflammation and pain - I'd suggest searching thru NutriLiving as we have several recipes and resources for these topics!
Good luck and keep us posted :)