I think you're right. I skimmed through the first 150 pages and found a few references to the hope of escaping Arrakis on frigates, and to the fact that Tuek had "fast ships" that could get them off-planet, but there's no mention of anything indicating that the smugglers could go around without the Guild. Your question about "but then what?" after Tuek's fast ships got them off-planet is a good one. What the heck did Jessica and Leto actually think would happen once Tuek had them on a frigate in orbit? Jessica seemed pretty sure that this was a legitimate escape option, so I wonder what her next step was.
This also brings up a question I'd never considered, which is - how the heck does anyone actually summon a Guild ship for transport? I cannot recall mention anywhere of FTL communications tech, and in the absence of that how could anyone actually communicate to the Guild to come pick them up? If the only source of FTL travel is through folding space, and if that can only be done accurately by a Guild navigator, then it seems unlikely that anyone other than the Guild could even communicate from planet to planet. Maybe the Guild makes regular stops at all colonized planets just in case they're needed or to ferry messages around? But then what of Great Houses in peril that need emergency evacuation, or of the Atreides fallback plan to escape Arrakis to which Leto refers?
Maybe the prescient navigators can foresee someone requesting their assistance and plan jumps to systems before the people who hire them even think of the idea. "We predicted you'd want to see us, here we are." "Whoa!" This would make being a kwizatz haderach a great annoyance since the Guild mailmen wouldn't be able to ever know if you wanted them there or not
That said, some of these star systems aren't all that far from each other. Canopus III (Arrakis) may be 310 LY from Earth, but it is probably within 5-10 LY from its nearest neighbor. Proxima Centauri, for instance, is only 4 LY from Earth. If Dune has such a neighbor that is inhabited then a sub-light ship going at even 0.25 light speed could get there using an initial thruster burst and then drifting in maybe 16-20 years, which is not too bad at all in terms of saving your House without Guild assistance. Maybe "fast ship" implies a decent ability to travel between close star systems without foldspace tech?
I got a bit off-topic here, but I guess I'm trying to show that Yueh was needed to ensure the battle would end very quickly and that neither Leto nor Paul could command their forces during it. He also ensured that Jessica would not pose a problem in capturing her, by giving the Harkonnens her precise biological details and arranging to have her drugged. Even if the Guild had intended to stonewall Leto's family had the shields stayed up, they still could have had time to escape to the Fremen or the smugglers. Eventually they could have perhaps gotten off-planet in disguise on a Guild ship once things settled down, through the smugglers or with help from the Fremen. If we assert that the Old Empire did have FTL data transmission then in a prolonged battle the Atreides could have simply contacted the Landsraad and it would have been over for the alliance. In the book Leto doesn't even know the shields are down until it's too late, and Paul and Jessica had already been drugged by that time, so there was no one in charge to send messages off-planet even if they had the tech.