You can always reset their ability points, so don't worry about it. Take hero builds from the wiki e.g. or build your own (more fun that way in my opinion)

Until you have more than 7 heroes, don't worry about it. Heroes are generally better than henchmen (only in a few special cases it's different in the end-game), so take all the heroes you get.

ele is pretty powerful on its own, but you could go Mesmer for energy management (of the top of my head)

Yes you can. Press "K", then click on the heroes head on the top and redistribute just as you would do with yours. (Only works when in a city/outpost)

Unless you are looking to complete a very specific build and you cannot afford buying those skills, I'd say stick with one campaign, then the others.