No. That doesn't matter. It's when the Borg discovers the Federation that matters, because they begin assimilation as soon as they find out. But to answer your questions, there's no problem with Starfleet not holding a news conference concerning the two missing outposts. And I'm not sure just how much Borg knowledge affects the dilithium markets in the Rigel system.
The Borg do not necessarily come in full force every time they see a new civilization. I imagine knowledge of impending doom in the Rigel System would at first raise the price of dilithium because ships would need to "refuel" to get out of there. But as the Borg drew closer and closer the price of all goods would drop as people would just want to get what they can and get the hell out of there.
If I recall correctly, there was a Strange New Worlds story where Data, having outlived virtually everyone, was stuck on a decimated Earth with the only remaining species in the galaxy - the Pakleds. He eventually tricked them into murdering him because he was tired of living. Or something like that.