If the testator had simply written "my wife" then the bequest would likely fail, because he had no wife at the time of his death. If he had written just "Stella Fenwick" then it would likely work, even though that was not her exact name, and "my wife, Stella Fenwick" would also work even though the extra description is inaccurate. There is no doubt about who was intended, and courts will overlook defects in naming. (This is assuming there's nobody else in Dr Fenwick's life with a similar enough name, such as a sister named Sarah Fenwick, which would create an ambiguity; or if he were married to somebody else so that "my wife" and "Stella" were distinct individuals.) Other testators have misstated the names of their own children, their siblings, or themselves. This is well settled; for an old example, see River's Case of 1737, which also considered whether "son" can mean an illegitimate son:
If a man is mistaken in a devise, yet if a person is clearly made out by averment to be the person meant, and there can be no other to whom it may be applied; the devise to him is good.
The principle goes under the grand Latin name "falsa demonstratio non nocet", "a false description does no harm". If the ambiguity is so great that it cannot be determined who is intended, then it's another story. The situation happens more commonly with bequests to charitable organizations, where the legal name of the entity may be different from the name that the testator uses.
It was clear enough that the solicitor could tell Stella Warren that she would not inherit: he understood that she was who the testator had named, albeit with some inaccuracy. There is a slight possibility that the testator meant to create a conditional bequest, i.e. make it effective only if they were married at the time of his death, but more express words would generally be needed in order to make this compelling. I think Stella Warren could argue her claim and the solicitor is wrong to dismiss this out of hand.
A similar case is In re Wagstaff , where the testator left his house to "my dear wife, D. J. Wagstaff; if she shall so long continue my widow, for her own use and benefit, and upon her decease or second marriage, then over". The lady in question was never married to him, and therefore could not be his widow, and in fact was married to another man all along. On appeal, it was held that "the testator means beyond all doubt, and it has not been disputed, the lady with whom he went through the form of marriage". The additional inaccurate words are not a problem.
In another similar case, Pilot v Gainfort , the testator Dr Frederick Pilot gave "to Diana Featherstone Pilot my wife all my worldly goods". He was not married to her at the time and she did not share his surname - indeed, he was married to somebody else, who had disappeared in 1921. The will was executed in 1927, and the couple were married in 1928 (since after seven years' disappearance, the former wife could be presumed deceased). In probate proceedings, the will was held to be valid - of course they were married at the time of death, but this does show a will where the name and description of the legatee were inapt at the time it was written.
The law as of 1965-1973 was not fully settled on what evidence could be taken to help interpret disputed passages of a will. There were many rules specific to the interpretation of wills, which were not obvious even to practitioners. So even though Stella Warren has an argument, she might have had difficulty persuading a judge, depending on the approach taken. This changed in 1982 with the enactment of the Administration of Justice Act. In its section 21, it brought in a general rule that for any ambiguity, "extrinsic evidence, including evidence of the testator's intention, may be admitted to assist in its interpretation". This allows examination of the solicitor's meeting notes, for example, which could illuminate what the testator asked the solicitor to accomplish in the drafting. Such "parol evidence" would previously not have been allowed.
Therefore, aside from the murder, Stella would have an easier time today making her case, though in both time periods her case is quite strong on the principle.