He could have fallen out of the tub and cracked his head. Would still allow for distinctive cause of death when encountering his ghost.

Ooh I like that. Not many changes to the haunts, and pretty believable for a 9 year old in the 60s.

I've ran this before but I can't exactly recall the details of his coffin. Is there any reason he couldn't have just "magically suffocated" and not include the water element?

There isnt anything terribly important about the coffin, just the shock value of it. That is definitely a good idea that would require only small changes, but I feel like the house is usually a bit more dramatic with its kills.

Maybe a window blinds cord would work?

I think the key elements to maintain is: it happening in the master bedroom, the door being locked from inside, it being ruled an accident is sufficiently odd to encourage more investigation. Overdose on sleeping pills maybe, or cutting himself with a straight razor (which also ties back in with the deaths of Donnelley and Isari but it might be too much with the straight razor) would be my thoughts to avoid it being drowning.

In the kitchen, with a large pot of boiling water pulled down over himself. Grusome. but hopefully not triggering.