the question is how far above the waterline is your shower drain ?
make sure you take into accounts things like load, rolling, etc... I wouldn't connect the shower drain straight outside unless it's at least 18" above WL.
also, if you are reusing the existing discharge thruhull, it shoudl have a rising loop which will prevent flow...
When you have a non working sump, attwood for instance, you can remove the pump and switch and install just about anything in there... much cheaper than puying a new sump.