If the Etsy trader made the ring then they could have made it without sending it off to be hallmarked...
In doing so does increase the price of the ring.... I believe both metal and stone would need to be hallmarked,
A lot of small traaders make their own rings via whatever choice of metal so do put in the hard work, skill and graft to make something beautiful, but to save them costs and the customer costs won't get it hallmarked.
The seller might have even been scammed themselves regarding the stones...
I'm sorry this has happened to you, but personally I wouldn't necessarily buy such an item from Etsy, and if I did, I would get in writing that due to such things happening, I'd have full rights for refund if after getting said item valued and assessed and turns out its not genuine .... That I could claim for a refund and return the item...
There's so many loop holes here, and time is against you...
It's been altered as well.
And in all honesty, the jeweller adjusting the ring should have checked and notified before tampering with the ring, they are in the trade and know how this stuff works....
Best of luck moving forward