Hi Andriy, it's good in some way that work has started, but I'm inclined to think that if your pictures show the limit of where they have cut back so far, then it's not nearly far enough. Indeed, I would be totally unhappy with any suggestion of continuing to use as a base any of the existing glass fibre within the damaged area, it's all got to be cut away. Anything less will be a skimped job, and that is exactly what appears to be their intention at present. You will know from the hammer tests how far the hollow sounding areas seem to go, and one thing you can be very sure of is that the damage will have gone back a lot further than you thought. As others have said, this is a job that cannot be skimped in any way. So the original glass fibre will need to be cut back and completely removed to a point well beyond the evidently damaged area in order to get back to completely sound and undamaged GRP. Then the edges of the undamaged GRP will need to be ground back further to provide a chamfered area upon which fresh GRP can be laid onto, extending further out with each layer until the new matches the old. I'm not well acquainted with laminating glass fibre, but I would imagine that epoxy resins would be used in preference to polyester, and that Kevlar or carbon fibres may be used in preference to glass, but be guided by your independent surveyor.
In your posting last year you mentioned that you paid the charter firm to provide the insurance, this is not a good situation because it means that you are dependent on the good will of the charter firm, their insurers and the boat yard to see that you get a good job done. Because all three of those firms will be looking to minimise their costs, you will have a major fight on your hands to get them to do what is needed rather than what they think is good enough. For this reason you need a really good and totally independent surveyor to help you fight your corner. Yes, it will cost you for your surveyor to be present, but you can make it very clear to the other parties that if your surveyor does not accept the repair as being entirely satisfactory, that you will not accept the repair. For this reason you also need independent legal representation. On completion, and assuming that the job is done to everyone's satisfaction you need a guarantee for as long a period as you can get that the job is sound and that the boat is seaworthy.
Next you need to consider that your boat is now damaged goods, and that when you come to sell it, that your prospective purchaser will more than likely walk away from any deal once they know about the keel repair. That means that your boat will be unlikely ever to achieve as good a sale value as an undamaged boat. So your independent surveyor should advise you on completion of the repair with their best estimate of the value of the repaired boat, and their estimate of the value of an identical but undamaged boat. The difference is something that somebody will need to pay for, but I wouldn't like to say whether that should be the charter firms responsibility or that of the insurer, but no doubt your legal advisor could point you in the right direction. I wish you good luck, you will need every bit of it.
Remember, don't trust anyone who is connected in any way with the charter firm, their insurers or the yard, and keep your wits about you in regard to your own surveyor, because it's not unheard of for corruption to occur.