In all of the info on installing magic latern it says not to use battery grip. Only with original battery. I can;t find the battery compartment door. Can it be done with the grip? Has anyone done this?

Loaded fine on my T2i with camera grip. I am 100% sure it will with the T3i also. Go on load it you know you want to and you will not regret it.

When I use my T3i I always record audio with a separate recorder.

So as I don't need audio monitoring on the T3i, is there any benefit to be had with ML?

What are the pros and cons?

It's a bit fiddly to use sometimes, but it has endless toys to play with. Practically speaking it's worth it for Focus Peaking (even though it's not deadly accurate), Zebras, False Colour and the Colour Histograms.
They are all very very handy. Suddenly your cheapo DSLR has a lot of proper video camera features and you can fly on instruments, so to speak, instead of squinting at your screen in the sun and hoping for the best..

Totally worth it. Shortcuts for 3x zoom on the t3i, iso, and white balance are great. Also gives you iso levels and color temp levels that are not available otherwise. Makes the camera much more user friendly in addition to the pro tools it adds. Also things like changing the bit rate can come in handy.

Just don't expect Focus Peaking & Zebras to be anywhere near as good & useful as they are on a proper video camera. I know that many swear by Magic Lantern but personally I find that it overcomplicates the camera as there are too many features that are OK-ish but will never be as nicely implemented as if Canon built them properly in the first place.