I concur with Wolfgang that as long as your DP sets up the audio well for the interviews you should be fine.
Doing the one man band thing only really gets tricky when you have multiple people talking at one time eg. a dinner scene with lots of people or a hike in the hills with lots of people etc etc, any of these "verite scenes" with multiple people is when an audio tech with boom/multiple lavs is worth their weight in gold.
The camera mic will be fine for b-roll but for an interview thats more than a few feet away it wont be much good. If you take it off the camera and mount it on a stand close to the subject being interviewed it could be a good addition to the lav mic too.
When you go the one man band route its best to acknowledge that it can be done but if and when something goes wrong it definitely takes one person doing multiple jobs much longer to identify problems and correct them than when you have a dedicated sound person doing nothing else.
I shot one man band in a verite scene last week, a demonstration by workers who met at a parking lot and marched a half mile to their factory. It was sub zero temperatures and all was going well right up til the march got underway. The second they set foot on the road the lav went down. I had to march along and try and fix it on the fly. It didnt happen and we had to go with camera mic only until they arrived on site and we managed to rectify. Thats the downside of one man band cause if we'd have had an audio tech he probably could have fixed the problem while I shot on. With only me I had to choose between fixing it or shooting and after a few fumbly minutes I had to give up.