Welkin is not a perfect character, but that's what makes him interesting. He's too idealistic and naive for his own good. Although in the anime he's too ditzzy.
The reason Alicia calls him a liar, is because Isara is Darksen. Since Alicia didn't seem to completely believe Welkin's story, Isara's race caught her by surprise.The Darksen as a race are persecuted and abused because of certain preconceptions (kind of like jews and gypsies). In the game Alicia and Isara knew each other (Bruhl is a small town afterall), and I'm not sure why they changed the script so much.
I think they attempted to mimic the style of the Rendered cut-scenes from the game, but didn't manage to pull it off.
It was their hometown, and they fought to ensure that everyone evacuates safely.
I like the game's storytelling much better, it's more political and deep. In game, the Militia is sent to take the bridge as the Cannon Fodder. So Welkin's mission was not just about a stupid bet, but survival and necessity. In anime the way he carelessly charges into enemy miRABt just to prove himself makes him a much more shallow character.