Derivatives are a violation of copyright. The safest path would be a license to make derivatives of the original images.
1. These aren't derivatives.
2. They are simply inspired by what they are replacing. Although close, they are different enough that they should be safe.
3. BW/SN and the BattleTech IP have historically had a good relationship and I see no reason for this to suddenly sour. After all, the former actually helped develop the Japanese BattleTech release.
4. Macross has had so many new series that the old images practically aren't even used anymore.
5. BW/SN would have to reach across the Pacific to pursue anything legal. These are different markets and BW has no direct distribution of Macross over here. There is no way for these IPs to be in opposition of each other.
6. It's a moot point and no one in the know seems nervous about such legal actions.
EDIT:
7. From what I can tell, Big West/Studio Nue and Tatsunoko were fine with the art in the 25th anniversary art book, which is why they were included in the first place. I still have the original PDF of it. It was set to go to print until Harmony Gold said threatened legal action despite the original companies being okay with it.