For reference: Scootaloo's army(the one marauding through your southern lands) that disappeared had used Agartha's OP spell, paths through the deep, in order to move back to my lands as I had seen Niefelheim's army about to move on me. Also, garrisoned (I.E. those troops in the top) will fight in battles so long as there is a commander present, and the PD commander counts. Although they stay inside a fort.
There is actually a whole lot of me using that spell in this war-- any time you see my army pause for a turn or dilly dally attacking unimportant provinces instead of important ones it tends to be them waiting for reinforcements to come in. That is why you see my army attack above Tien Chi on turn 30 and then attack the capital for good on 31-- there were troops warping in during that turn to reinforce the assault. (Also, IIRC, I was hung up for 1 turn to reach a research goal that I deployed during the assault)
As I think I've already mentioned a dozen times before-- the most powerful asset of Agartha is using paths through the deep to cut supply-lines and reinforce my army. A great illustration of this is the fact that during this video you can basically see all the troops I owned-- there was nothing but mages back home researching. And since you had a lot of your resources tied up in both the amphib army and in province defense-- if all else was equal I was bringing more stuff to the fight.
Also, I think you skipped over one of the more interesting battles, or maybe it was on turn 27.... but there was one battle coming in where you deployed fire arrows, but the archers were on fire archers so you didn't get to see much of what they would have done to my giants aside from maybe a round or two of fire. The battle was a bit of a beating, but it had a tiny glimmer of hope that could have saved you if you had noticed it-- Had you preserved one of your F3 guys from that fire site in your capital and recruited a ton of archers set to fire largest with fire arrows it could have done massive damage. The spells I laid down in the capital assault were almost more about killing the 60+ flaming arrow archers that I feared than the reality of what it did kill.