Sorry to break it to you, but Joshua's AI actually will not attack Natasha, and the enemies around him charge you, so the easiest and most risk-free way to get him is to slaughter the enemies around him, and then recruit him.
Joshua's supports are fun to read. Even if you don't plan on using either of them, Joshua and L'arachel's support conversations are the funniest support conversations ever, and they give some pretty good bonuses, like +3 attack and +15 critical. A few others are pretty funny as well, and he has a love interest in Natasha or Marisa. Another useful support to utilize of his is his support with Gerik, which together they can annihilate whole armies while standing next to each other. He has useful supports with a bunch of people, and most of them are fairly quick. I usually make him a swordmaster, but whatever you want.
It is a reward for saving all of the villages.
Can you even count 5x as a gaiden chapter, I mean, its not even optional, its just called a gaiden for some reason.