We recently played that quest and it felt quite unbalanced favoring the OL. Are the reinforcements in encounter 1 meant to be the whole rat swarm group at once, as written? Or 1 at a time as usual?

Thanks !

Here it's just 56% in favour of the OL, so pretty balanced: http://d2etracker.com/stats_quests.php

And yes, the reinforcements are the whole rat swarm group, up to the monster limit.

Let me guess, the heroes lost too much time fruitlessly killing rats? 😜

there was a valyndra agent up ahead and the rats were no laughing matter since they could "surge" feast and could attack from the first round!

Well, it may have been the Valyndra agent that tipped the quest in favour of the OL.

Sometimes the heroes need to assess the situation: what's more beneficial, actually winning the quest or salvaging as many search tokens as possible and/or defeating a summoned agent?

As an OL, having your agent defeated is quite the setback, so I only use it when losing it is less bad than losing the quest...

About the rats... yes, they can hit hard (they can also hit like a wet noodle half of the time), but in that quest the heroes can't afford to waste time killing or even weakening respawning rats, unless it's with AoE effects.