"x and y were the only suspects in [some list of more than two rooms]"

[Martika14]

For a clue such as

Julius and Charles were the only suspects in rooms touching the west wall

just sheer logic does NOT seem to suggest that both Julius and Charles must be in a room touching the west wall - after all, not every room must contain a suspect. Yet if I don't make that assumption, there isn't enough information to solve the puzzle. Can someone explain how the logic actually works? (This was the second puzzle in a row with this type of clue.)
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[Afterburner]

There is no logic nor assumptions involved. The clue is telling you straight up that those two suspects are in rooms touching the west wall.

In your puzzle, there are three rooms touching the west wall. Julius and Charles are specifically in two of those three rooms. You have to use the rules of the puzzle and the other clues to figure out which two.

The clue also lets you know that nobody else is in a room touching the west wall. So the remaining 3 suspects must be in other rooms.

For this specific puzzle, it is obvious from Clue #3 that one suspect (either Julius or Charles) is the killer, and the other suspect is in the easternmost column of the library. No one can be in the Larder because that would leave the murder room empty. And no one can be in the westernmost column of the Library for the same reason. So the suspect in Column 1 must be in the murder room. And there is only one tile in Column 2 which belongs to a room that touches the west wall, so that is the location of the other suspect.