How does the "frontline" work? Is baldur and the summons the only ones who can be on it? Or do you decide who's front line and who's not at game start?
"Frontline" is mostly a skill of Balder. Meaning that whatever is the next attack of the enemy team, when he is on the Frontline, he takes all the damage.
It falls to the enemy to decide which heroes he will attack.
For the summons, they are only protecting their master. So they take the damage of an attack when their master are namely targeted.