I have two questions regarding rules.

First, development cards that state "During battle,...", are they ment to be used once per battle, or can they be used multiple times per battle (if there is enough tokens on them, that's it)?

Second, for Dandelion, one of his development cards allows him to skip the monster obstacle. If there is foul fate token in the region together with monster card, can he skip the monster with that card and just pass control to other player, od does he need to take foul fate token?

Each of the development cards will tell you if they can only be used once, otherwise, you can use them so long as you have development tokens on them. I am assuming you are talking about Dandelion's ally cards (Cahir for example). If Cahir has 3 tokens on it, you can use all 3 to add 3 swords to your check.

For the second question, it specifies that he still must encounter the foul fate token if any are present. If it doesn't specify it on the card, then they might have stated it in a FAQ somewhere else.