Tech stagnation, space magic engines and incompetent eugenic babies, to name a few, are things that come baked in the BT cake, and are acceptable to me, it's fiction. However, there are limits to hand-waveuism that I'll accept, and I personally could not accept an interstellar civilization without reliable communication. HPG communication is and was never dependent on Comstar they were just an organization that ended up with a monopoly, so they do not matter too its continue existence or use, but an interstellar civilization as large as BT requires a (FTL) HPG communication network to function. It is one thing if the HPG never existed, but it was established as an integral part of the setting early on, allowing said Comstar to amass its' power and wealth. Telling everyone now, "yeah, that FTL comm network... you don't need it going forward." Smacks of plot laziness and increasing hand-waveuism. Since it was needed before. Which the OP rightly alluded to. ...
Not confusing reality with quasi-sci-fi, but no major civilization ever maintained itself without a reliable communication network. BT should be no different. ... .
I can definitely understand this. The Blackout should have far more ramifications than has been let on. For one, Mech manufacturing infrastructure. It seems, from what I recall out of the source books, that the Hegemony spread out different aspects of Mech Manufacture, and the houses did nothing to really change it when they took over hegemony territories. With FTLComm, you know how many MechWidgetIIcs you're getting and when on what shipment from Nearby, verses Cockpit component RadioIIc* from FarFarAway, which all go into that Warhammer you're cranking out on VaultWorld next to your capitol.
Suddenly, you don't have that timing anymore, and however many you get is what comes on the shipment. If Nearby sees a delay for some reason, but FarFarAway happens to get ahead, you're going to have an uneven number of components, and a drop in output to boot.
And, this is just broad strokes. It's much more complicated than that, but it's enough of an illustration, I hope.
And, Julian's traffic? Nobody should know where he 'is', but only where he's been, maybe. Pony traffic can be unreliable, especially when port authorities start pointing guns at ships and saying, 'you're not going anywhere', like Liao did from one system while they conducted their next wave of invasions of the Fed Suns.
But, on the flip side, big invasions would be very difficult to coordinate, requiring a lot of advance planning.
So, while I get the interest some people have in the Blackout, the current set of fiction has handled it poorly. Even the Dark Age stuff didn't really bring it to its fullest. Those subfactions that we saw in the early MW:DA game? They shouldn't have spanned worlds. That conflict should have been on one! And, then move to another the ones that failed in their bid for power flee to the next world, or we see similar groups form on another world and a different conflict is resolved.
Each world is effectively on its own in the Blackout, and any information it gets is about as old as pointing a telescope at one of the nearest stars and observing the incoming light.