No, I never believed they were purposely imported to control ladybugs, makes no sense, since ladybugs are not considered a nuisance, for the most part, actually I've never heard anyone complain about them. Furthermore, I know some gardeners actually buy them to place in their garden to combat the aphids (sp?).
I don't believe that either (but just guessing). Since ladybugs eat primarily little insects and stinkbugs eat produce. I don't believe they really compete for any thing. As for them swarming, the schedule of the ladybug and stinkbug swarms are probably totally unrealted.
As WOO said and from what I've read they, like many other things, were accidently introduced.