So, I think there are pretty good arguments for either approach to getting the Scylvendi into position to attack the GO. As far as going around the Neleost, there would be several disadvantages, the way I see it. One: time. The GO is busting ass to get to Golgotterath before winter sets in. If the Scylvendi are meant to attack them soonish, I'm not sure they could get around before deep winter hits. They would probably be delayed until spring, which would probably be too late for Golgotterath if they were needed as reinforcements. Two: at some point, they will encounter the denuded land that both the Horde and the GO trampled over and decimated. They would have no food or forage, and wouldn't necessarily have a Horde of sranc to eat to sustain them. Additionally, they would probably run into that situation as winter is setting in, making forage even more difficult.
On the other hand, Cnaiur was one of the principals in the Holy War that supervised the crossing of the entire army across the Sempis, arguably one of the greatest rivers in the Three Seas. I also don't think there's a bridge over The Leash, but surely he, of all Scylvendi, could orchestrate such a crossing with great rafts, barges, ferries, something of the like. He didn't seem to have any compunctions about crossing water, as he did it many times on scouting raids and such, before the Holy War actually crossed. I imagine the Consult
would employ Quya to keep the things steady and/or to propel them across the water. Similar to Kellhus' Raft, but arguably easier since these would be floating on water, and not in the air.
I think my two cents were spent long ago.