I can see no obvious problem with the piece based on the photographs, however given the quality of some of the copies on the market one cannot be obviously certain without close physical examination with a powerful glass.
The Imperial Russian market is in turmoil while 6-8 years ago when the market was at its crazy heights it would have made £50,000 without much difficulty. Now you would do VERY well to get a third of that figure, however I would suggest Holmasto ought to know as they have a history of selling quite good Russian material, based on your enquiry it seems you are suggesting that the auction house has no idea of the market. As a fellow member of the auction business I have found them quite professional in any dealing I have had with them and if you don't trust them, as your post seems to suggest, or you would have been happy to take their advice, why are you entrusting them with the auction?