Erewhon doesn't need to actually do anything about Lacoon II (or whaterver it is being called in the transportaion community like Manticore Travel Ban). What they should be doing is placing a Travel Restriction Notice into the notices on the update broadcast and advisory to any ship heading for or comming out of their wormholes telling them that Manticore has closed all of its wormholes to ships of SL (or SL Member) registry and no SL flagged ship will be permitted to use the wormhole at Hennesy and will be turned around.
I would expect Erewhon and Manticore both might use the Hennesy/Terra Haute bridge to trans-ship goods to and from SL flagged ships. Erewhon could certainly use it to advantage since their merchant marine could pick up a lot of traffic into the League. What this would do as far as exposure of the mm if the SL decided to use it as leverage to move a force to at least Terra Haute is another question.
Depends on what Manticore is doing at Hennesy beyond maintaining a wormhole picket/task force to defend that end. There is also the question of what the SL thinks they want to do about Hennesy- both the system and the terminus.
If the SLN were to go after Hennesy, they really only have two ways to go about it. One would be to send ships Joshua to Erewhon then to Terra Haute and go hyperspace to Hennesy. Of course, they MIGHT (if they were smart) figure that when they showed up at Erewhon (hell, at Joshua), even IF either of those two systems would allow a naval force (other than one show-the-flag ship) to make the transit, they would surely send messenger by courier or on the next of their own flagged ships to 1st Erewhon then on (by a courier who is jumped way up in the queue) on to both Terra Haute and Hennesy to let the governments and Manticore know it is coming.
The other way would be send the SLN force through hyper-space as they did with Fillerta. That way they would have at least a chance of surprise.
Manticore can certainly use the fees (not to mention the trade both ways that it can generate from within the Empire) from non-SL flagged ships picking up the prohibited ships cargo.
With the problems ongoing with Beowulf, the SL needs to think long and hard about taking on the systems involved in the map displayed route via Joshua. Joshua may or may not have a significant SDF but Erewhon certainly does (outfitted in great part with "Manticore Lite" equipment and they were an active and experienced component of the MA so their Navy (which is much more that a SDF) had a lot more real combat experience than the SLN. Experince in fighting with capital ships

. Erewhon also probably has its own penetration of SLN and, certainly, the SL political system. Their defence would not just be military strength. They would have a wide range of political responces available and bring pressure, probably in the very areas that the Mandarins are most vunerable. Blackmail comes to mind.
Besides, it appears to be allied with the SL, not part of it. For the League to come after/force a fleet or two through the wormholes, is going to have serious reprocussons with the rest of the League Members and any other allied systems. Think of this being a case of throwing firebombs at some of your neighbors and friends to and all the others are now making the determination that they may be next. Really bad move