Having problems with armatures and weight paint

Ok, I’ve been watching and studying a bunch of tutorials on armatures and adding weight paint to a mesh when its deformed from the armatures. But I’m having a slight problem that I dont know how to get around.

Basically its a game model I’m making. A lizard. I modeled this lizard and left a blank opening all the way across his mouth so I could add in bones and spread the mouth apart when the mesh was finished. But the whole point of that was so I could make the interior of the its mouth, then join the 2 meshes once thats complete.
Problem is when the model is skinned and weighted I cant take it into edit mode because when I do that the entire mesh is back to the way it was when I made it, the lizards mouth is shut. But clicking back onto object mode and the lizards mouth is open again. So thats doing me no good if I cant add the interior of his mouth if its shut again.
Also,… If I add bones and weights to the mesh, deform it a little, then go to join another mesh with it things go wrong. For instance if I click the interior of the lizards mouth first then Shift+click on the body and join the meshes, the interior of the mouth will deform and go all whacky when I move it. But if I select the body first and then the mouth and join them the mesh will return to its original form and I lose all the weight information that I worked on.

I guess what I need is a way to edit the deformed/weighted mesh in edit mode. And a way to join the interior of the mouth to the original body of the lizard without losing weight data and without having the interior mouth go all deformed when I move it inside the weighted mouth of the lizards body. If there is a way to do all that. This is driving me nuts.
Hope all this doesnt sound too complicated.

So I’m greatful for any answers anybody can toss my direction. I appreciate it.

A general rule of thumb is to finish your mesh before adding armatures / shape keys. The armature is a modifier to the mesh - the only way to then get the mesh to copy the armatured shape is by applying the armature modifier) and then, no more armature.

You have a few choices.

Go back and finish the mesh after removing the armature from it, then add the armature now that the mesh is finished

Model the mouth separately, animate it independently, make it a child of the head, link the armatures (e.g. action constraint to have the teeth open when the jaw bone opens). You might also just be able to have it affected by the same bones.

Duplicate the armature, open the mouth, apply the 1st armature (so that the mesh now has an open mouth), finish modelling, see if adding the second armature works now that the first is gone.

That’s al the useful suggestions I have.