May I suggest you check the alignment of the viewing and taking lenses in the first instance? It's possible the composition of the lens elements has been tampered with, however the focus discrepancy you described in the group photo could be very easily explained by the likelihood the viewing and taking lenses are not tracking together. In which case, you may have obtained a sharp image of the faces on the viewfinder screen, however if the viewing lens is a little off the true focus of the taking lens, the actual point of focus on the film will be slightly different. I mentioned this recently in my first post at APUG here and provide detailed instructions for dialling in the two lenses of a Rollei to achieve accurate focus.
Originally Posted by jspillane
It is of course also possible there is a focus misalignment due to the parallelism of the lens board to the camera film plane being out of factory specification. This is 0.05mm across diagonal corners, and is fiddly and time consuming to set back to spec unless you have some particular equipment to make the process faster and easier. If this was the case, you may (or may not) be able to notice that objects in the image at the same distance from the camera are variable in sharpness across the frame (Eg. one side sharper than the other). Hope this assists.