Although you can get any captured characters back, they will all be completely useless, unless they were captured in the last few chapters or were extremely overleveled.
As for not losing to all the Armours: Block the hallway with Leaf, Fergus or Brighton. They have the defence to take more than one hit. Leaf can also take out the Mages with the Light Sword and can recruit a character too (assuming you met the condition for it in Chapter 3). Before you open the door, hang around a bit to get extra Vulneraries from the Soldier reinforcements. The reinforcements are pretty fucking random, so be on your guard when releasing the prisoners (if they all survive and escape you get Chapter 4x, complete with another similar situation of being camped by a bunch of Armours and Mages, but also a powerful recruitable Mage with Grafcalibur, the Sety Scroll and a Hero Sword

).
Oh, and in Chapter 3, give as many swords as you can (3-5 Iron/Steel/Short/XL Swords maybe), as well as the Armourslayer, Hezul Scroll and a bunch of Vulneraries. The Fire Sword doesn't hurt either, but the only person who I think can use it then is Leaf, who has he the Light Sword. And the Lockpick. Don't forget the Lockpick.