In each example, property has been settled by the X settlement on the husband (H) for life, who predeceases his wife (W).
Remainder to W absolutely, who, by the Y settlement, settles her reversionary interest on herself for life, with remainder to B. Duty is paid on H’s subsequent death on the ‘settled property’, but there is no exemption on W’s death because she was competent to dispose ( IHTM04457) before she executed the Y settlement.
Remainder as H by deed or will appoint, and, in default of appointment, to W absolutely. W settles her vested but defeasible reversionary interest on herself for life, with remainder to B.
H died without having appointed, and duty is paid on his death. Even though W was entitled only to a defeasible reversionary interest during H’s lifetime, it was held, in Att Gen v Hay  2 QB 245, that she was competent to dispose. However, this decision is difficult to reconcile with the Coutts case, see IHTM04456 ( IHTM04456) and any situation in which you are considering denying exemption on the authority of Hay should be referred to TG.
Remainder to B absolutely, who, by the Y settlement, settles his reversionary interest on W for life, with remainder to C. Duty was paid on the death of H, and exemption is available on W’s death.
Remainder to W absolutely. H and W execute a deed bringing the X settlement to an end and resettling the property on H for life, remainder to W for life, remainder to B. Exemption applies on W’s death; she should not be treated as competent to dispose of the settled property on which duty was paid when H’s life interest came to an end.
Remainder to W for life, with remainder to B absolutely. After H’s death (on which duty is paid), W and B resettle the fund on W for life, remainder to B for life, remainder to C. You should allow exemption on W’s death, despite the resettlement, treating the fund on W’s death as the same ‘settled property’ that bore duty on H’s death.
Remainder to B absolutely. Duty was paid on H’s death, and at a later date B settled the property on W for life, with remainder to C. The settled property on W’s death cannot be equated with the settled property that bore duty on H’s death, so no exemption is due.
Remainder to W for life, remainder equally to B and C. After H’s death, on which duty was paid, B died bequeathing his reversionary interest to W absolutely.
If, on the principle of Fry v IRC  Ch 86, the settlement has come to an end so far as the half share derived from B is concerned, the exemption does not apply in respect thereof on W’s death. But if, on the other hand, that half share is still regarded as comprised in the settlement at W’s death, for example, if there has been no assent in W’s favour - see Rivington v IRC  Ch 149), W should be regarded as not competent to dispose, and exemption will be available on her death.
Cases involving decisions as to whether the life interest has merged with the reversion can cause difficulty. Fortunately they do not arise very often, and if you encounter one about which you are in any doubt, you should refer to TG for advice at an early stage.