One problem I see is that a high level cleric trying to turn a high HD undead creature could wind up giving it an obscene penalty to all actions.
When Bob tries to turn an 8 HD zombie (who fails his will save), he ends up giving it a -7 penalty to ALL ROLLS! Yikes! That means even though he hasn't turned it, he's basically rendered it helpless. Imagine a party with a 16th level cleric going up against a 16 HD lich - the lich is going to wind up with a minimum -8 penalty (minus turn resist, plus Cha, if any) when the cleric tries to turn it. This is if he succeeds his save too. Not good.
My suggestion is keep the mechanic for turn/destroy, but make the penalty static - perhaps equal to the Clerics Cha modifier (or just the -2 for being demoralized). (Basically, the penalty is unrelated to the sucess of the turn/fail of the save.)
BTW: I really like the +2 DC bonus for using a second turn attempt. Good thinking.