If the Worms decline the Rat penalty, they keep the ball with a 10-yard penalty assessed against them and an untimed down from the 50.
If the Worms accept the Rat penalty, we have offsetting penalties and the Rats replay the previous down.
If the Rats decline the Worm penalty, only the Rat penalty can be enforced, from the previous spot, with an untimed down. The Worms do not get to tack the Rat penalty on to the interception return.
If both penalties are declined, the game is over.
The most favorable outcome for the Worms would be to decline the Rat penalty, enforce their own, and get an untimed down from the 50.