As you say, a DOV is needed to create the trust if reading back for IHT and/or CGT is required. However, the DOV could be worded to appoint trustees of the legacy as well, thereby avoiding the need for a deed of appointment and retirement of trustees of the Will.
The settlement rules for income tax only apply to parents, not to grandparents. The original legatee will still be treated as settlor for income tax purposes, but provided she is excluded from benefit there should be no adverse income tax consequences.