question on importing sim models into blender

Hi.
If you will remove doubles on your mesh and transfer all weights, vertex paint and uv_1 from a suitable reference, it will not break along seams.
If you do need to test how an item behaves often, it would be faster and more reliable to just start the game and check there. If you have a lot of cc that makes the game load slower, move it elsewhere temporary.

You can use S4 morphmaker to preview morphs but it seems to not always be accurate.
You can rip sims with simsripper but they come out unrigged, so it is pointless for the purpose of testing since you will have to re-rig them.