Maybe. I assumed it would be somewhat like Artanis, since she apparently goes with your Origins Edition. I would say they are going to give early access to her when Overwatch comes, then release her properly a week or so later. But this is just speculation.
Meh. I don't like this SC trend when they turn generic, literal non-entities into heroes, while important lore characters are completely ignored.
About Mengsk, based on his datamined ability names (Construct Propaganda Tower, Post Bounty, Xel'Naga Artifact) as well as his non-combatant status, I would say he is going to be some really weird extremely long-ranged / global Specialist, somewhat like Abathur. I would imagine a more static gameplay (meaning building stuff and fortifying positions) with him compared to Abathur. Propaganda Towers would likely buff allies / debuff enemies in their area or have other more elaborate abilities such as mini-nuke booby-traps (possibly through talents). I have no idea atm about how Post Bounty would work. What's certain is that we need more unique and strange hero concepts, since they help to differentiate HotS even more from its competition (also they add a lot more gameplay diversity than regular heroes).
Yeah, Alarak has become one of my favorite SC characters. He is a really amazing take on the normally mind-numbingly LG/LN protoss.
Fenix would also be nice to see, but Alarak has my vote if it comes down between the two of them.
And why not have the best of both worlds? Let's have a SC event with a SC map AND Fenix, Alarak and Mengsk!