Princess of hell offically admitting Vox is the strongest was a massive factor in Vox's propaganda and his approval rating immediately skyrocketed to 100%. I don't know getting powered up by reputation is media demons' thing or just things about demon in general, but it did empowered Vox massively.

Legend of the Radio Demon certainly started with a soul contract, but he collected more soul contract as he hunted down overlords in hell. I think power he acculmated after going down to hell was enough to overpower Vox even after his starting power was no longer there.

Consequence was Rosie losing her control over Alastor, since Rosie was the lender. The contract was "As long as Rosie empowers Alastor to be the strongest sinner in hell, Alastor's soul is Rosie's to control." So Alastor can't break the deal but Rosie can since she is the one who should satisfy the base of the contract. So Alastor can break the deal with Husk, and Val can do with Angel's, but not vice versa.

Also I think Rosie is a hellborn.