As for idiot proof, what I mean (and thought I articulated in the initial post, but apparently not) is which inputs and outputs should be used.
Ahhhh, I see now. I thought you were wanting something in place of the T1 rather that to use with it. As the T1 has a 1/4" output (balanced or unbalanced), I would suggest the $40 iRig2 guitar/instrument interface for the iPhone (and select androids). You're going to need some kind of cable adapter anyway with built in resistive attenuation and the proper wiring to convert a mono 1/4" input to the iPhone's 3.5mm TRRS and they typically run around that same price. So, it'd be either a 1/4" to 3.5mm TRS cable to a 3.5mm TRS/TRSS adapter cable or a plain old 1/4" guitar cable to the iRig2, which plugs into the iPhone. (Note: if you have a lighting connector, you'll need a lighting/headphone adapter as well.)
The iRig2 is unpowered, small, designed to do this, takes a simple instrument cable connection from your T1 Main Out, and unlike a simple adapter cable, gives you headphone monitoring of what's going into and playing on the iphone (if tracking), input volume control and a 1/4" output to an external amp or speaker if you want. If you normally use the T1 1/4" output to your L1(?) you can easily put this between your T1 and L1. If you use the T1 Tonematch output, you can connect the unused Main 1/4" T1 output to the iRig2 and iPhone.
Hope this helps,