Rosie teasing about how Charlie was Alastor's girlfriend before saying how he's "an ace in the hole" is a reference to Vivziepop saying how despite Alastor being aromantic asexual, he is not aware of the modern labels and just thinks "he has not found the right person" yet.
Alastor makes a deal with Charlie, being very specific that he will ask for a favor in the future; it will not harm anyone, which is why Charlie agrees. All deals in hell are binding, so the wording must be exact as possible. Alastor knows Charlie won't fall for a trick, so he carefully chose to ask for a vague and benign repayment.
Cannibal Town has strict fashion code. Anyone not dressed in the style is marked as food. Charlie is safe doubly as princess of hell and the fact her tuxedo passes the code.
When Charlie first tries to win over Cannibal Town for their upcoming battle against the Exorcists, and Susan always interrupts Charlie, Charlie briefly starts to sing "Inside of Every Demon Is a Rainbow" from the pilot.
As Rosie offers Alastor information on sinners he can make deals with, she is likely the reason he gained so much power in the past. Rosie always keeps an ear open for opportunities.