You make some good points. I think the argument that the drafted QB can be a bust holds true in either situation - whether or not we get McNabb. If he is a bust, we will not find out till year 2 or 3 or whenever McNabb is done. By that time, we will have built an overall good team, McNabb's career would be done and we will have a big hole at the most important position. In the other scenario, if we draft a QB, he starts NOW and he is a bust, we will still have time to salvage the situation as the rest of the team is built. But the long term potential of a decent QB is huge.
My point is that if we think a can't -miss prospect is available in the draft, take him. We do however have to prioritize shoring up the OL this year. If we don't, then we should keep the QB on the bench and let the serviceable guy keep us going. Bringing McNabb behind a suspect line is marginally better than this situation, I agree, but we lose out the draft pick we give up for him. Now if we are likely to get him for a 4th rounder or lower, then perhaps there is a case to be made (which is what you said in the last line also)
So, in summary, I think we need to look to boost the OL for sure. And if we manage to do that, getting a QB in the earlier rounds may be a good idea.