for CPU vs CPU, I use Pro-10 to tune the stats in the most realistic way possible. For example, in my Nadal vs Federer both have around 90% on average in their skills.
When I test by myself, I also play again a Pro-10 CPU as it's supposed to provide the most realistic experience.
However, everything in the range of Junior-1 to Master-10 should give decent results.
If there are balance issues at Incredible-1 or above, right now I really won't care about them, as I still didn't finish to tune & build everything so it wouldn't make to worry about the Incredible level as it's really special compared to the other ones (ie: it has superhuman reflexes and boosted skills).
Strikeware203,
1) yes, now there's a Deadzone per player instead of 1 global Deadzone.
4) If I need to test something without the CPU looking for the win right away and send balls more to the center, I usually pick the Junior-10...

5) Yes, it'll have night matches
6) There's no way to change the crowd density