Jim,
Both questions require some judgement to assure the accuracy of your results. Here are my thoughts:
1. It's quite feasible to measure the out and back leg with the same accuracy as any other course, then simply add that distance to the overall course length. If you're comfortable with the exact route of the original course and you account for any distance that may be lost as the runners leave and return to the original course, I don't see a problem. On the other hand, riding a 10K measurement doesn't take that long and could serve to verify your original course.
2. If I could add 35m or more to the course using a steel tape, then I would take the second ride and if it compared within the 0.08%, then I would make the adjustment to the longer of the two course measurements. However, if I didn't have a straight section at the start/finish where I could make the adjustment with a tape, I'd do two more rides from the approximate location where the added 35m would put me.