I recall that where a will seeks to dispose of residue twice, it is the first clause that applies, on the basis that once its terms are complied with there is nothing for the second clause to act on. I also recall it was probably a 19th century case that deal with the matter.
Despite the muddle between absolute and trust interests, the testator appears to have wanted to benefit A and C only. It may be that with the benefit of advice from counsel of 10 years standing, you would be able to treat A and C as the sole beneficiaries and allow them to direct how the estate should be distributed (possibly by them executing a deed of variation to verify the terms upon which they share the estate). I appreciate that if A and C would not be the only beneficiaries under intestacy, such a route could be disputed by those otherwise entitled, but the willingness to argue against such a distribution might be down to the strength of your counsel’s advice.
Whatever the outcome, in scenarios such as this I believe counsel’s advice is the safest way forward, assuming the amount in question is more than a couple of thousand pounds.