BGV apparently expects to travel some 1200 miles to Guarnak during the icy winter, using thousands of Snow Lizards and thousands of caribou besides at least 10,000 gene modified Morgan horses, which seem more like the Russian steppe ponies, but even at half normal horse rations for the Morgans and the same for the caribou, with the snow lizards getting a weight-proportional ration to the dragons, carrying a month's worth of such rations [~250 tons per day] would be the maximum he could move and have it exhausted by the time they reach the Kalgaran river, so how do they get supplied traveling down the 600 Guarnak-Ice Ash Canal?
From the textev implications, I suspect his advance guard is traveling much faster than the 20 miles per day max we've decided is their maximum rate.
While Wyrshym has dismissed Ranshair, suppose that Ranshair while having only a small garrison, had a dozen or so extra sailing ships unload the supplies BGV will need, but more importantly, suppose they also unloaded ice boats.
While the church thinks its the only one able to afford the steel for the runners of its ice boats, Howsmyn is making a lot more steel of a lot better quality, and could provide the steel for supply type ice boats very cheap.
Size-wise, the CoGA Lake Pei design is based on the hull of the CoGA messenger galley, apparently [best guess] only around 100-200 tons or so, 'displacement' or mass.
The main projected ICA iceboats are intended for supply not speed, though smaller ones for scouting and troops transport are also obvious, and could likely be much bigger than the CoGA's using more and better runners to distribute the weight [catamaran, trimaran, or more runner designs], so they might deliver up to 200-300 tons at a time.
While the wiki article author is apparently ignorant of ice racing in the 19th century including races all the way to Albany from New York and back in 24-36 hours, with some racers in the 100 ton class, with around 1000 square feet of sail, being able to deliver supplies up a frozen river would be the primary advantage the ICA sees.
Thanks to OWL etc, such large ice transports seem quite possible feeding data through the college and the ship designers in the inner circle.
While the Northland gap is about 200 miles from Allyntyn, and St.Zhana is about 200 miles further, the high road crossing the Kalgaran to Fairkyn is only around 340 miles from Ranshair which the supply ice boats could easily reach in a day or two, with possible winter moon light being very useful for navigating after nominal sundown.
Having established an 'ice head' there, they then might start sending supplies to BGV before his main body has reached the Kalgaran river if they're running low, then they approach and surprise Fairkyn using the ice boats on the canal to advance far more rapidly than the TL's believe possible.
Thanks to the ice boats they might then cover the remaining ~540 miles to the Guarnak-Sylman Canal in a 5day or less even if using only daylight [8-9 hours at that latitude?], far faster than Wyrshym's army can march back to the city, besides cutting the Guarnak high road to Five Forks.
So BGV's real military secret might be he can be supplied and move far faster than anything Wyrshym knows is possible.
Granted there are navigation concerns on such a narrow canal, but the potential rate of advance appears to be the only solution possible.
I invite your comments and look forward to the positive ones.

L