What is it like as a 12B Combat engineer?

12B is what you make of it. I did four years, got out, then went to college. I know guys who did 20 and one was CSM of the army when he retired.

My first unit was armored engineers, no Blackhawks. My second unit was the 326th Engineer Bn in the 101st Airborne. I was Air Assault qualified and rode a lot of Blackhawks and Chinooks.

I also went to Airborne school as it was in my contract.

A lot of what you are asking depends on where you end up, which depends on the needs of the Army.

You can be a 12B your entire career if you choose. It's definitely versatile enough. There's also an engineer officer branch if you choose to go that route.
Flying in Blackhawks is highly possible. While deployed we fly in Navy Seakings and Chinook.
Airborne will be dependent on your unit. If they are an airborne unit you'll go to airborne school. If not you can request it. If you compete in the Best Warrior competition, a yearly event, the winner gets their choice of schools to go to, which you could choose airborne.

To answer the opening question, it's awesome! Close to 13 years as one now and it's been really fun.

1) longevity is heavily dependent on your mindset. If you go at it with a positive outlook despite any drawbacks that may happen, you could find yourself retiring with 20 years of good experience under your belt. If you have the attitude that pounding pickets sucks and let that govern the rest, you won't last very long. And, your unit leadership will have a significant effect on all of it.

2) There aren't any MOS's that you don't have the ability to fly in a Blackhawk, or any other troop carrying airframe.

3) Combat arms, in general, typically get the opportunity to get Airborne, Air assault, Pathfinder, etc. easier. However, in addition to those schools you can also get Sapper as an engineer while other MOS's can't.

Also, the type of component you join can have a large effect on your career mindset. Active vs Reserves vs Guard - all have their own pro's and con's.

Hope this helps!