Installing the bay at Eris

Here is how one might set up the Cartesian Bouguer problem, at relativistic distances and nonrelativistic times-of-flight. The "pirate" pursuer expects to detect the merchant's infrared at maybe 1 AU distance - here the retardation due to C is eight minutes.

The big dumb freesailing merchant can be a colony-ship off to Alpha Centauri, with hibernation. Starshot at 0.2C is 60000 km/s for a 20 year trip to that system; I allow the merchant 120 years at a negligibly-relativistic 10000 km/s. Assume the merchant reaches that velocity in the Kuiper Belt around 60 AU from Sol (and Earth): where the pilot and crew cut the engine (saving propellant for the decel), and all take naps.

The pursuers, meanwhile, start by assuming G-force, so are using robots. These are humans not leaving our system; they know that they cannot issue on-the-spot commands from light-hours away (eight of these, if from our inner planets). They have programmed their robot fleet to intercept the merchant's general course. They had to do this far in advance, before the merchant even left; further, they knew their robots' trajectory would burn some propellant. So, they found a KBO which would approach that path approximately. The KBO being icy would supply the additional propellant, to intercept the merchant.

Also, the initial speed is... high. So the initial acceleration is high; maybe a coilgun. 1000 m/s²? For 10000 seconds...? KBOs move negligibly compared to a colony ship (Kepler, yo!) - Eris at 68 AU goes only 3.434 km/s. And (mostly) at right-angles to the merchant - although at 0.4 ellipsis, in the middle of its orbit there's some angle. That's the sort of KBO at the epoch of time the pursuer will want.

All this done, the pursuers now have their robots ready to catch the merchant when it arrives in the not-so-near vicinity. The robots' AI is rudimentary: the orders are "fill the tanks; wait; pursue". The pursuers aren't being clever about anticipating the merchants' flight because, what if the merchants change their trajectory (not easy at 10000 km/s admittedly). And one excellent reason for a constant speed (which is >10000 km/s) out here is that it is a maximum speed, to protect as far as possible from ambient Kuiper snow-specks.

As for the pursuit's overall motive: yeah, it's difficult to justify a robbery this far from a decent fence. A pure saboteur can work; lots of crabs in a bucket dislike when a crab gets 67 AU from the bucket. Better: they're not even pirates, but are - exactly - the propellant-supply mission, to feed more ice to the colony-ship for that eventual final decel.