"You need a lab to tell them apart yeah."

No, in general, you don't need a lab to tell them apart when they're in the raw state. Fossil ivory is pretty distinctive in appearance.

You could maybe stain modern ivory to make it look like fossil ivory, but that would make it considerably less valuable.

And yeah, you could label the containers of modern ivory as fossil ivory, but you could also label them as medical supplies, soybeans, or wingnuts. It isn't going to fool anyone who actually opens the container.

Are you an expert on this?

No, I'm not an expert, but I've seen plenty of artifacts that are made from both fossil ivory and modern ivory. They simply don't look alike, not even to a casual glance.

How many exampes of both have you seen, in person?

I won't dispute that fossil ivory could possibly be bleached to look like modern ivory (which would have no effect on the elephants or arguably a positive effect, since it would be eating some of the market for the modern stuff), or that modern ivory could be stained to look like fossil ivory (which would devalue it).

I just don't see any economic incentive to do either one.

The situation only holds when the market price of mammoth ivory is roughly the same as that for elephant ivory. If mammoth ivory was much cheaper than elephant ivory, then killing elephants wouldn't be economical. Unfortunately, mammoth ivory is an extractive process, like mining, so it is unclear if the price could be brought down and kept there for an extended period. Still, it is more than likely that mammoth ivory has had a protective effect on elephants; although, I know of no studies examining the issue.