I also use a two piece system and have had good luck with it. Recently modified it a bit and increased wear time again. Burping is easy to do with this if necessary, and you will notice that different things will cause more gas: ie. drinking carbonated beverages, drinking thru a straw, chewing gum, etc. As well as certain gas producing foods. I cut out carbonated beverages a while ago and it made a huge difference for me.
I am very short waisted at 5'5". I wear the 12" length bag but I fold it into 1/3's and it lays very well again my abdomen. I don't wear hipster pants - my stoma placement on a short waisted person just won't allow it comfortably. Pressure against the stoma after you heal will not hurt it... just not too tight.
And, yes, you will be able to lay on your stomach tho I find myself starting out in that position and ending up on my side usually. I have an egg crate mattress pad on top of my pillow top mattress cuz I like a soft mattress, but it sure makes laying on my belly more comfy.
Please ask any quesitons you may have. We can only share our personal experiences. Remember everyone is difference, heals in their own time, and develops their own routines.
Do you have an ET (enterstomal therapist) to talk with?
You are very right about finding your own comfort levels in sleeping, sitting, etc. Everyone heals at a difference pace and you will find that you will become more comfortable the further away from your surgery date you go. Just do what feels ok, but try new things too.
Re: your skin being dry to apply the wafer --- this is very important to get the best seal of your wafer. I know that I have mentioned on this forum in other places that I use Viva paper towels during my appliance change. I fold each one into 1/4's and use like a wash cloth and then just pitch them with the used wafer/bag. I use one to wash, one to rinse, and most importantly for me, one to dry. The paper towel really absorbs the dampness on the skin well and has worked for me since 1984!!! Just my hint! They are the most expensive paper towels, but you gets LOTs of changes out of one roll.
Good luck to ya and keep us updated on your challenges and accomplishments!