We're told in Busman's Honeymoon that Peter always visits the condemned men whom he's helped bring to their current position, and that he asks for their forgiveness before they're hanged. (Then, of course, he proceeds to go home and have a nervous breakdown). So, are we to assume he did this for Urquhart, or attempted to do so? It's hard to picture Urquhart consenting to see him, but was even Peter having a breakdown over his death, considering that Urquhart had not only committed one particularly calculating and nasty murder, but had very nearly succeeded in using the judicial process to commit another one? Not that these sorts of things work rationally, but I wonder how that particular night ended up playing out.
And while the question "Did Philip commit suicide?" is obviously a red herring from the start to give the reader a little breathing space before zeroing in on Urquhart, I did find it bewildering how much time the characters spent pursuing that angle without ever remembering the fact that Philip had at least three months' worth of arsenic doses in his hair. It's natural that Harriet, who knows she didn't do it and can't imagine how (or why) Urquhart would have done it, would fall back on thinking it was suicide on the basis of "If you have eliminated the impossible, etc" but Peter and Charles Parker spend two or three weeks trying to chase down that sodium bicarb packet without either of them saying "Hold on a moment, is it really possible that this blighter deliberately poisoned himself no fewer than four times?" Bunter remembers -- he talks about it when he's having tea with Urquhart's servants -- but otherwise that fact goes straight down the memory hole for about 200 pages, until the issue arises again with regard to what they might find in Urquhart's hair. All I can conclude is that Peter was somewhat off his game for emotional reasons and Parker was humouring him :).
Of course, since Peter and Harriet have both read about Madeleine Smith, there's always the possibility that they subscribe to the theory that Emile L'Angelier actually committed suicide -- writing those dramatic diary entries and telling his friends "If she poisoned me, I would forgive her" in an elaborate scheme to frame Madeleine for murder. There's also a similarity in that Madeleine's first documented purchase of arsenic didn't occur until after L'Angelier's first attack -- though unlike Harriet, she never claimed she was buying it for a book. However, even Harriet doesn't seem that convinced of Philip's suicidal tendencies, so perhaps it's as well that they don't draw the L'Angelier comparison after all.