I have to say you are showing the patience of a saint with these people. They seem to be under the impression that you have to prove your claim to them at this stage. You do not. You have asserted your claim to copyright and it is up to them if they feel aggrieved to test your claim in court. You did not de-activate their site; you merely asked for the infringing image to be removed. If their hosting service chose to take the whole site down, then iwrite's immediate grievance lies with the hosting service.
Anyway, since you have gone to these lengths, I sincerely trust they will see the futility of their position which rests entirely on undermining your claim to copyright.
Just to be clear, I think there can be no dispute that assuming your wife is the legal heir to her grandfather's copyright in the painting, she has the capacity to authorise you to make a photographic copy. So let's look at the UK law on the ways in which copyright may be transferred between owners.
Section 90(1) of the CDPA 1988 says:
Furhermore since the painting was not published during the artist's lifetime, section 93 also applies:90 Assignment and licences.
(1) Copyright is transmissible by assignment, by testamentary disposition or by operation of law, as personal or moveable property.
It appears from the details you have already provided that even without the evidence of the wills (ie if intestacy had occurred at any stage) the normal operation of the law would have resulted in the painting and the copyright in it being passed down in much the same manner as you describe, that is to say down the blood line within the family. It would be very hard to see how any other claim to the copyright in the painting could arise. Indeed I suggest that in the face of documentary evidence you hold, the only thing which could trump that would be an assignment signed by the artist before his death transferring the copyright to someone else. Clearly if he done that, it would not then have formed part of his estate to be passed on as directed by his will. But no-one has seriously suggested that occurred, so I think we can assume that a court would have no difficulty in upholding your wife's claim both to title in the painting itself and the copyright attached to it, along with any other copyright work of her grandfather.93 Copyright to pass under will with unpublished work.
Where under a bequest (whether specific or general) a person is entitled, beneficially or otherwise, to—
(a) an original document or other material thing recording or embodying a literary, dramatic, musical or artistic work which was not published before the death of the testator, or
(b) an original material thing containing a sound recording or film which was not published before the death of the testator,
the bequest shall, unless a contrary intention is indicated in the testator’s will or a codicil to it, be construed as including the copyright in the work in so far as the testator was the owner of the copyright immediately before his death.