I don’t think that HMRC will agree with the advice that you have been given that the period of administration of Mrs B’s estate is ‘not concluded’. See my post in the thread linked below.
From the time that the residue was ascertained the son in Canada should have been declaring his share of the dividends to the Canadian tax authority and the UK son the same to HMRC. There may be little or no IT due by the UK son’s estate, especially as HMRC can only go back as far 2018/19 absent the making of a case that the UK son or someone acting on his behalf brought about the loss of any tax carelessly or deliberately. For the period of admin R185s should have been issued and the income thereon declared by the legatees. But absent bad conduct the UK son can’t be assessed.
If the shares are sold, the son resident in Canada is not liable to UK CGT, but may have a liability in Canada. The executors/legatees of the UK son will have a capital gain based on the shares’ probate value on his death.
Executors income tax liability during administration - #4 by jack