If I were to hook my computer up to an amp (using Logic Pro as an amp simulator) do I need to run though the amp and cabinet (guitar -> computer -> amp -> cabinet) or can I just plug the computer directly into the cabinet? (guitar -> computer -> cabinet). I'm hoping to just be able to use the cabinet...

I'm thinking of using a Marshall MG412.

you will need an amp of sorts to power the speakers in the cab.
You would need a solid-state amp (tube emulation compresses sound just like tubes, so modeling->tube is bad) that’s actually designed to accept something other than a guitar as an input. Tech 21 sells “Power Engine” brand cabinets with built-in solid state power amps for this sort of thing.
You'll need a power amp between the computer and the speakers.

Basic signal chain:
Preamp (laptop) > power amp > speaker
Admittedly you could use an amp with a line in and the EQ set flat (don't run it into the main input and don't run it into a tube amp)
ohai little sig.
i'd use powered computer speakers (or better yet powered monitors) or else you are going to need to bypass the speaker/microphone emulators
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
^ i would agree. if your band has a PA, i would run it into one of the line inputs and use the PA as your power amp and speakers. or powered speakers, or a power amp and unpowered speakers.

but you need to have a power amp in there somewhere.