From personal observations:

1. It definitely depends on the parameters of the person entering the Dragon Utopia and their army: if the dragons aren't as afraid of the retaliation from strike stacks, they will slaughter them. The calculation is based on dealing maximum damage while receiving minimum retaliation and the ability to survive after it. If instead of waiting with angels you hit, say, green dragons and took a hit back—as a rule, expect a second strike from red ones, which will take out at least one angel. However, if there's a level 25 Tazar with 30-40 defense, the dragons will even hit single units and will never bite into a stack that is lethal to them. As this criterion weakens, the "bravery" of the dragons increases, including the "tastiness" of the leech at higher levels.

2. As for the placement of units in the Dragon Utopia—no matter how much I read about it—it didn't matter one bit. No matter which stack I placed the leech in (the one the dragons bite in the first round), they would hit it—as long as they could reach it :)