Signing Mikkelsen for full season "from nowhere" seemed strange and high risk.
Who knows how Rally1 change would work for him, we have many drivers that struggle (Evans, Sordo, Breen), and some that don't (even old ones like Loeb).
Lappi hasn't shown drop in performance in Rally1, so is clearly a much safer choice and Hyundai need some "safe" drivers. (Specially since Sordo's speed dropped so much).
Part time drive in 3rd car is another question altogether and depends a lot on the driver combinations. There the question is whether Mikkelsen wants to drive for Hyundai if he gets offered only 4-5 rounds, I am not so sure. With 7+ it is likely different.