I don't find it likely that UP-s would be within the UA shell: with folks harvesting UA-s from there for Prof. Palin, they would have been found already, unless the spawn rate is astronomically low. In which case they might as well not exist. I do find it more likely that they'll be somewhere between Merope and the shell: that would be the prime candidate to go looking, what with them also pointing towards Merope. However, looking for them close to planets is a pretty good idea. Personally, I've almost exclusively looked in deep space or in orbit of stars, but checking near bodies would be a good idea. After all, where would you send probes first?Finding Free-floating UPs: I now suspect that these might be found in Anomaly SS close to life-bearing planets in the UA shell (close enough for the name of the planet to be seen in the lower-left part of the HUD). The UPs seem intended to find life, so perhaps they will be attracted to these worlds - this is my hypothesis, although I have no idea how long it might take for the right Anomaly SS to appear. Reaching the next stage of the puzzle clearly seems to require having the UP (unless you are incredibly lucky, see below) so I would hope that it would be a wait of minutes to half an hour, rather than hours.
Especially if the system map/barnacle finder theory turns out to be correct.
One more thing to add to your theory though: terraforming candidates are designated as such for humans. For other species, habitable worlds could well be different, and they might send probes to search for those instead. For example, from earlier lore it has been pretty well established that Thargoids live on ammonia worlds.
I've yet to come across any ammonia worlds either inside the UA shell or between it and Merope. However, from the Earth-like list, there are five ELW-s in four systems (one system has two, in a rare configuration of the two Earth-likes orbiting two different stars) and if I'm eyeballing this right, all of them are within 100 ly of Merope. So, some places to check:
Pleiades Sector IR-W d1-36 and d1-42
HIP 16671 and 19637
Also worth noting that an Alliance listening station was built a while ago orbiting an Earth-like world in HIP 19072. It's also "in range" of Merope.