Thanks for the translation - the majority of the grammar and wording is generally not bad but as noted above, you've really gotta watch for your subjects, especially if you're going to translate a mystery novel. Just firing some examples from the major scenes:
16.1 - 'Before 11pm, I arrived at the house and guided him to the study' -> 'Just before 11, he arrived at the house and I guided him to the study' - She's obviously not the one arriving. It's late, so 'just before 11' emphasizes that it's late without being ambiguous.
16.4 - 'What is the content of your conversation' -> 'What was the content of their conversation?' - Obvious from context, especially given:
17.1 - 'Since they haven't entered the study' -> 'Since I didn't enter the study' - the two men clearly entered the study! She must be talking about herself.
18.1 - 'Now then, next... it's my daughter, Catherine' -> 'Now then, next... I'd like to speak to your daughter Catherine' - I don't think the 16-year-old female detective has a daughter.
18.4 - 'I went to my usual room' -> 'I went to my room as usual' - This is important because it means she regularly retreats to her room -> there's regularly DV happening (also 18.5 should ideally be 'As usual?' because she's echoing what Catherine is saying and it scans better)
20.1 - 'Unlike my husband, that person goes straight home' - 'Unlike [Mary's] husband [of the house], that person goes straight home' - I doubt the maid is married to the dead guy.
Edit: given that Edna's husband does return late in the story, this could be taken to be her husband. However, right after this is when she starts ranting about the dead guy and his habits and debts, so I'm pretty sure this is meant to be Mary's husband (the dead guy) she's talking about
25.2 - 'Aren't you just repeating what you heard from my sister and the maid?' 'Yes, I am' -> 'Surely you've heard this from my sister and the maid?' 'Yes, I have' - MC wasn't the one talking.
40.1 - 'I punched the colonel' -> 'you attacked the colonel' - MC is the one giving her deposition
41.5 - 'An even more intense violence than usual began' -> super literal. 'his temper overflowed more violently than ever'.
43.1 - 'But it seems there is evidence, doesn't it' -> 'But is there any evidence of this?' - he's clearly trying to argue
50.2 - 'The maid Edna muttered that it was 'rust from her own body' -> 'The maid Edna muttered that 'you reap what you sow'. You can't translate 身から出た錆 that literally.
I went back and skimmed the first chapter and you did good there as far as I can tell, but it kinda stands out here where the text is fine but doesn't really relate to what's going on.
Also, lmao at 'check the river for the dumped stolen goods', taken straight from a Holmes case