Hi lovely I'm a jeweller (silversmith/goldsmith)
If the silver weighs less than 7.78 grams, which it most likely does, then it falls within the exemption weight and doesn't legally require hallmarking. However we are required to display our dealer notices in print if in a physical shop and online too, so my website shows mine, does the seller display a notice (if they are UK based they need to)
In terms of the stones I would get a second opinion from another independent jeweller.
The problem you have here is the time frame for the returns period has passed also did the other jewellers resize the ring? If they made any alterations without you returning to the original maker that would too pose a problem.
One option you would have is to see if you had a legal case for the stones. You need to weigh up the costs, how much was the ring compared to how much it would cost you to take it further etc
Sorry you have had a terrible experience hope you get it sorted soon. If you have any questions I can help with feel free to ask x