can this be a carry on?
Unlikely as the bags dimensions without the day pack are 28"H x 15"W x 14"D for a total linear dimension of 57" which exceeds most airlines limit of 45 linear inches.
YES! I've used it as a carry on for several flights and not had a single problem. It fits in the overhead area better than many rolling suitcases. Here's the thing - it is technically too big but my roommate and I both have 65L bags and never had problems on planes.