Will bad play in one cruise line's casino put booked trips at risk?

Potentially. This is pretty rare and is determined on a case by case basis. Most players we book are consistent when it comes to their play. But anytime you don't play on a trip, it will affect future offers. 

With URComped, your trips are based on a rolling average of your play. So if you played well on 3 trips and on your fourth trip, your play was down, it would bring your average down, but not drastically, depending on the case. The more good trips you have the less one bad trip will affect your offers.

If you don't play in Cruise Line A's casino and you have 2 more comped trips booked through Cruise Line A after that trip, then they may look at your future trips and reevaluate the comp. They'll look at how big the investment is on their part.  There's not like a hard and fast policy on it, but it could be concerning and would definitely affect future URComped offers if play dropped off.