Haha, I have that one and The Majestic, for sure. Can't remember any others off the top of my head. But Arsenic is the last one I bought, well after the initial release.
To be fair, only a handful of scenes take place anywhere besides the house's drawing room, so that probably helped a lot. If it were made today, there would definitely be a lot more locations and we'd absolutely be shown the basement. It's a testament to Capra that he keeps everything visually...
Yeah, I think Massey did just great, but it's so obvious that the Karloff bit is intended as a meta-joke that it would have been perfect if he'd been allowed in the film.