1. HB has a tripwire should the code it has implemented be looked at by Warden, HB would be shut down from use.
2. Can you be banned for prior botting? Yes. Wether it's due to logs, monitoring or Blizzard discovering a function HB calls and then going back to see who's client called that function in the past few months etc.
3. Does Blizzard have a special way. No and yes. Warden. They rarely perform large updates on it, when they do it tends to be before new major patches. But as far as I'm aware Warden can still be changed on the fly and given set instructions to look into something.
4. Should you be worried? No. I've not suffered a loss or any trouble in my few years of using HB. The only trouble that has ever occurred with Buddy products was an unfortunate period where the old GatherBuddy used the same function as another bot/hack. Blizzard went after this hack, GatherBuddy was caught in the middle unfortunately and I do believe the tripwire was non functioning at the time. *This was VERY long ago*.
As it stands, HB's protection has been solid for a long time. There have been no banwaves recorded at all. Yes there are bans, usually due to user error or reports, not the program being flagged.
Also if it helps you feel safe, the current lawsuit against Buddy Products. Clearly Blizzard see's the Buddy Team as a force to be reckoned with.
That should answer things.