The course consists of a short segment down the upper vista followed by 221 or 110 laps around Drumheller Fountain for the marathon and half marathon respectively. The two events will begin at different times and use different start lines. Runners planning to complete fewer laps are welcome to join the half marathon start.
Excepting pacers, full marathon participants must intend to complete the full distance.
Unlikely. Many athletes train and race on smaller radius tracks without issue during indoor track season. There is evidence that consistently running the same direction on small indoor tracks over time leads to strength asymmetries which likely cause injuries. Alternating directions introduces rest and is probably sufficient to avoid acute issues for most runners.
You may also take comfort from knowing that indoor track marathons have been held for years without event.