I visited Heide Park last year in september. We stayed in the 'Holiday Camp', which has a dedicated park entrance (next to Scream). We started our two days with Dämonen and then Bobbahn and Limit and we then just made a tour around the park. I recommend to do these rides first, because right after park opening, the lines are usually short for these rides. I wouldn't go to Krake first, since it is the first big ride visitors pass when they enter the park via the main entrance.
On the second day of our visit, it was raining like hell during the afternoon, but it stopped raining around 3pm. At that moment, the park was already dead (it was a weekday and no school holiday, so before the rain it was already really calm at the park), but all (major) rides stayed open until 5 pm (1 hour before the official park closure).
It was so calm when it stopped raining that I could do 11 consecutive rides on Dämonen without me having to leave my seat once between 4 and 5 pm.
I have no idea at all. My prediction is that it will depend on the weather that weekend. The park you visit on the best day will probably be the busiest...
We usually go the Heide Park first, since it is the closest to home for us, so we can have more sleep on the night prior to the trip.