I'm continuing to look with suspicion on Miss Hadow, and I discovered what is either a typo or a significant clue. On p. 226 of the Berkeley paperback edition, she says "I'm Mr. Scrudder's secretary." On page 243, she says "...in Mr. Scudder's office I just come in and go out whenever I please." I read an Agatha Christie mystery where an apparent typo like this is a major clue. Does this same name error occur in all editions?
If this isn't a typo, she called her supposed boss by the wrong name. There is a Charlie Scudder, an executive vice-president (p. 135). Den drank with him in the early days of his presidency (p. 152).
Here are two possible explanations for the slip:
1) Hadow is an impostor. Weer says he doesn't recognize her (p. 226). Between his first meeting with her and the later one, she remembers (or looks up) the correct name. This would work if she is the Tilly ghost.
2) More interestingly, "Scrudder" could be a slip of the name she privately gives her boss. In the Urban Dictionary, "scrud" can mean "To harm, kill, or by omission of action cause to be harmed or killed, another living being." Suppose Charlie was a fellow conspirator with Weer in the coldhouse prank, and Weer rewarded him with a vice-presidency when he inherited the company. If Hadow is related to the coldhouse victim, this may be her way of thinking of her boss as a killer. In this case, vengeance for this death may be her motive for killing Weer.
I suspect she poisoned Den's last drink. If she isn't a ghost, she can't have poisoned the first drink on p. 220. Miss Birkhead was at her desk when Den entered and nobody else came in until he called for Miss Birkhead and Miss Hadow came in instead. There was a bit of a delay, and Den had to ring twice (p. 226). Did Hadow kill Birkhead, or decoy her out of the way to get access to Den's office to poison him? Time-slippage makes the situation fuzzy, but I prefer to think of Weer's entries into the past as self-consistent where possible. It would explain the sudden secretary replacement without notice. She wrote Birkhead's death notice herself, so it need not be true (p. 243).
Why did she barge into the office when Den was meeting with Dan French with a trivial request that could have waited (p. 243)? She might have overheard on the intercom that French was having a drink with Den, and rushed in with the first excuse she could think of to save the innocent French if necessary. Something may have reassured her that French was in no danger. French was drinking whiskey on the rocks. Den at least sometimes drinks whiskey and ginger ale highballs (p. 152). Maybe the poison was in the ginger ale.
I realize this is a lot of speculation from a little typo. Does anyone agree this may be significant?