So many variables to consider, but I think you may be overthinking this issue:
1) Does McDaniels even end up leaving? He's paid handsomely, has a great rapport with Kraft, Belichick, and Brady, and is experiencing a once in a lifetime opportunity. He didn't take any interviews last week, and some of those positions have been/are being filled. It would appear as though JMcD isn't quite as concerned about leaving as some may have you believe. Hopefully, he won't have the opportunity again for another couple of weeks, and by then even more positions will have been filled.
2) It would depend on whether Lane Kiffen would even want to leave. He's coaching with (arguably) the best college coach that there is at a major program which is always in the thick of the championship hunt.
3) It would depend whether Bill Belichick would want him here. He's been a bit of a controversial coach, and comes off as a bit arrogant to me. Is this the kind of coach that Belichick normally targets?
4) The difference in systems obviously comes into play here.
5) We don't really "need" an OC, per se. At least not in the sense that they can't wait a year or two to groom/choose their replacement.
6) If McDaniels leaves, my guess is that we'd promote someone from within who knows the system well.