Oooh! I love this topic!

I assumed hey have sensation. Have you ever had trapped gas and it hurts like a motherf*cker? But really, I don't know. I have dissected before though!

As for the method of opening, that depends on you and your story. If the victim is being dissected then it is a vertical cut and you need to go around the navel. You can find dissection advice on the internet. Actually, you can find whole dissection kits! You can even buy animals preserved in formaldehyde to practice and learn from! But I assume you don't really want to go that far :D.

The abdomen is thick though. If you've ever dissected a fetal pig in high school (I did, and a shark... And a mudpuppy... I named the mudpuppy Cartman the sparky Parker Fluff Gills)...anyway, I digress. Organs are held together by a protein membranes and connective tissue so they don't just ooze out like a mess of noodles. Honestly, it's kind of hard to describe now that I think about it. It's not like they won't start messing up and falling apart, just that you discover a lot is connected so, you pull on one thing and it lifts the rest in a messy awkward fashion. And, it can smell. Particularly if you cut into the intestines. And I don't mean like a fart... I mean like.... IT SMELLS.

Oh! I just remembered something about pain actually...I only know about kidneys and the liver. These are very vascular organs so they are very very sensitive to pain, more than any other organ in the digestive system or urinary system. I'm excluding reproductive obviously.

Anyway, I hope that helps, and hopefully someone else can give you a better answer for the other questions.