Even if all hits of NAir connect, you will still be at a decent disadvantage. It's a gutsy move/cross-up, and people that play Bowser tend to like gutsy options, so YMMV. DTilt is a block string, but it won't be safe against some of our worse MUs, like Sheik. YOLO FAir will just get you punished while you're still in the air or while you are trying to fast fall the move to shave off frames.
1. Jab 1 is our safest move on block, though you still want to avoid hitting a shield with it anyway. Obviously, if they're scared enough to shield, it's a better idea to try grabbing. If they're trying to bait an approach, then you're probably better off just standing around and being intimidating. Throwing out bursts of Fire or fully spaced FTilt/DTilt is a valid option against slow characters or characters with slow-ish punish options
2. Eh, if you've saved double jump, you can usually just escape to the ledge or avoid jank follow-ups completely. In most cases it's fine as long as you avoid habitual instant double jumping, ADing into the ground, or whiffing your YOLO Bomb. Our fast fall isn't completely bad, so use it. Bowser's aerials are not safe to throw out unless you know your opponent is over aggressive, and even then characters like Yoshi, Marcina, RosaLuma, etc will just beat our options out.
This is not accounting for conditioning your opponent, which a few of our members enjoy doing when switching DAir for Bomb.