Regarding the money calculation – I bought a hero (from Dungeon) to bring in the elves, but after the purchase, I had very little money left, and Kiry had enough turns to reach you – there were enough turns, but I was shocked by my financial calculations, and the time ran out((, I should have given the army to the hero from the Dungeon, and Kiry would have attacked, not bought a hero, but used magic and the next day bought the elves and attacked (there would have been slightly fewer elves than you had, but 8 marksmen would have taken them out, and there wouldn't have been a morale penalty from the unnecessary Dungeon unit).
Regarding discovering the army in the Dungeon – I always have and still believe that you can use the fixed map editor during the game. I often see the map for the first time on the day of the game, and this time I just glanced at which castle would be better to choose and took the Elves for a rush... and only the loss of the hero on the 3rd day and my subsequent blunder didn't even give me a chance to win...