Did the guy at DJI completely understand what you meant by a skin as I had seen 3 separate sources (fb groups and another forum) with screen shots of convos with DJI support stating the same and even had one myself to confirm as I was skeptical. To me, as I politely discussed/argued/prompted for a reason, the chap said that it was not a problem attaching small labels with drone registration or personal details but full skins were not acceptable. So, wondering if perhaps your guy may have been thinking of just stickers etc.
I was surprised tbh as I do not believe it was ever mentioned with regards to the Phantom so perhaps they changed their policy a while back. I assume the answer you receive will vary with whoever you get from DJI on the line or chat and quite possibly it may not be correct that they void warranty, perhaps they have changed their policy in the last 3 weeks to allow skins but at this stage until I see an official press release or similar I would not risk it. Despite what you may be told one day they can clobber you the next and even with proof from a DJI chat session you could find yourself with a hefty bill.
Chances are if your Mavic goes in for RMA then it will be down to damage, so you would not get the skin back anyway.
It would be great if we could get a definitive answer either way but any type of escalation with DJI does not seem to get anywhere and would we necessarily trust the final reply anyway, well unless it was an official statement by DJI.
It is of course up to every individual if they remove the skin prior to sending it in for RMA but personally after what I have seen I am not going to risk it.
