It could use a little trimming and tightening, but not too much.
I feel leaving motives in an unexplained state really adds to a sense of dread and paranoia. After all, that's the very definition of fear- we are afraid of what we don't know.
Yes, Walter does feel shoehorned in a bit, and that's another thing that makes CT so ready for adapting- 100% accuracy isn't necessary. As long as the main plot and characters are intact, little bits removed or added here or there will likely improve upon what is already a great story. However, you could argue that in the context of a film, where more background and story can be added, stuff like Walter's appearance could be justified somehow.
The thing that I love so much about playing Clock Tower is the fact that, if you go by one of the longer and more satisfying endings, it can be completed in the span of a movie running time. It's a perfect movie-length story. Obviously characters, especially the four orphans, will have to be developed and be given their own unique personalities, but that goes without saying.
I'm not a huge fan of slasher films for numerous reasons, so I'd prefer it to be a more atmospheric slow burn movie. This would entail limited amounts of gore- gore isn't shocking in the slightest, and only makes a film feel more cheesy and desperate, and horror's obsession with it is just so childish- and keeping the villains and monsters in the dark, so that their few brief appearances are actually effective. I'm not entirely sure what constitutes as a slasher film anymore anyway, is it just having a killer with a weapon picking off people one-by-one? I personally think the usual tired slasher formula would be a disservice to Clock Tower's incredibly unique story and flavor.
So yeah, the films I mentioned all have great aspects that would lend themselves to a great adaptation of CT. Ti West's The House of the Devil and The Innkeepers are extremely slow burn, making the scary moments infinitely more effective. Jennifer Kent's The Babadook is extremely atmospheric and character driven, the titular monster appears throughout, but the limited lighting forces the viewer to imagine what the monster could be, rather than having it fully displayed at all times. David Robert Mitchell's It Follows, while a bit overrated and overhyped, has extremely realistic feeling teenage characters and a great sense of dread and is a great example of using the spirit of a slasher flick to make something way more intelligent. Films like James Wan's The Conjuring or Kubrick's Shining are great examples of the setting taking on a character of its own, as well as having overall great production value and mainstream-ish appeal without sacrificing any filmic integrity. And finally, Robert Eggers' The Witch is a film directed by a production designer- it looks superb while being extremely under budget,with no need for big-time celebrity actors, and is apparently going to be the creepiest or scariest film of the past two decades.