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I asked about this fairly recently, and the answer I got is that it does indeed work. As long as the CTB ability is not specifically phrased to refer to "this/his next Rescue Attempt," you can replace your blocker with a CTB ability in a side-battle.
You miss the point. Asahel is not bringing the character into the side-battle, but the main battle.
A choose opponent ability used before a blocker is presented acts as a restrict ability that allows your opponent to block with the selected character (if they would not normally be able to) and restricts them from blocking with any other character or choosing not to block..A choose opponent ability used after a blocker is presented acts as a return ability that returns all characters your opponent controls in battle to their owner's territories and brings the selected character into battle. The return and band abilities in this case are mutually dependent; if one of them is not or cannot be done, the other cannot be done either.A choose opponent ability is instantaneous. A choose opponent ability targets both the character(s) to be removed from battle and the character to be added to battle (and the blocking player if no blocker has yet been presented).