You can't do syncing with the FR2le as far as I can tell. If you're set on a busman mod though, you could get the busman Hd-P2 instead. As to the SBD+AUD mixes, distance away from the board has nothing to do with it. You're not mixing together thru a mixer at the venue, you'd be mixing in post. So by syncing the clocks, all the samples will line up, you'll just have to find the common point to start them at (eg, find a snare hit near the beginning to line them up). With your mics further back, the aud source will have more of a delayed start than the board mix, but once you'll line up the sources prior to mixing together, so the amount of this delay will not matter.
4ch is also good if you do onstage recording. Center DIN/XY pair, and then some split omnis to add in, or use the additional two channels for spot mic'ing (like a keyboard that is far to the side or something else that might be low in the mix like percussion -- congos, tablas, etc).