There is no reason the proposed arrangements cannot be put in place using a deed of variation, provided the 2 year window is still open.
If separate trusts are to be used for the disabled son, the daughter and the grandchildren, I would usually recommend that they be established separately and the variation merely direct the relevant percentage of the estate to be held upon the individual trusts.
Provided the variation is in place before the property is sold, I see no reason why an appropriation cannot take place to share the gain between the entities. However, it needs to be born in mind that the annual CGT trust allowance will be shared between the 3 trusts, effectively meaning that only 1 trustee’s annual allowance will be available against 60% of the gain. If the sale will complete before the executor’s annual CGT allowance is no longer available, there might be a benefit in appropriating only to the brother and his son before sale, with the executor’s allowance being applied to the remaining 60%.
Going forward, if separate trusts are set up, during any tax year in which they still exist they will share the trustees’ annual CGT and basic rate tax allowances equally, whether or not there are gains, or taxable income, within any individual trust.