Spoiler:

I'm suprised the police never noticed the lack of a human skeleton in the ashes? Don't the police have a doctor on the case, it must take a very insense fire to cause bones to break down. Also what was Oldacre's long term plan? Just turn up & say it was a big mistake? or as a long lost relative? Can you get a clear cast from a wax impression of a fingerprint that will make a clear print. This was written some years before the first conviction using fingerprints.

Most likely Oldacre planned to flee the country under dead of night w/whatever funds he'd managed to stash. It's been awhile, but I think there might actually be a scene in the story in which Holmes deduces this.

Good point about the bones, though. Evidently Lestrade's specialty wasn't forensics. (Albeit fingerprints - while not convicting evidence on their own - had been recognised as a powerful ID tool at this point. Doyle's a little ahead of his time, but not much.)

According to the Annotated Holmes, while it might take some careful doing, yes, you can take a clear wax impression of a fingerprint - I believe many sculptors use/used wax for similar purposes.

IIRC, he actually planned to move to another community where he had set up a new identity, having made several large monetary gifts to that identity.