On the specific issue of the Steiner/Davion wedding, according to the BattleCorps story
Unholy Union Hendrik Grimm III was aware of it well in advance and made his own plans.
For a pirate realm to be viable they need to have good intel to pick their targets;
access to a HPG is almost a necessity unless they have extensive courier JumpShip networks (or well-connected agents).
(
Edit: On second thought, probably not. They only need to be within 50 lightyears of another HPG to receive its messages, as a HPG is only required to send messages but not to receive them although ComStar would still have to set up the sending HPG accordingly and know when and where to receive a message, and how to decode it.)
Jordan Weisman said in an interview that the slow message exchange was an underlying requirement for the neo-feudal setting.
But given the possibility of real-time HPG communication I suspect the real reason is that ComStar needs time to read and digest all the off-world messages.