Man, does Petherick come across as a stuffed shirt, doesn't he?
This is another one where I'm torn - on the one hand, it's another change of pace from the murder mysteries, also trying to sort of force a few cards on us, while the real answer is something different. (ie: there was no physical switching of the will, it was just some other means that made the writing disappear.)
On the other hand... disappearing ink? Really? *sighs* *shakes head sadly* Plus it's a bit harder to enjoy a story told in such a... dry style as the lawyer's.
I would also note that he seems to be awfully careless with such a legal document as a will, leaving it lying around in an unsealed envelope in other rooms, with other people around. Especially when he knew that there were people nearby who would vastly benefit from said will disappearing. Then again, I guess maybe since he didn't like the will himself, he might have been (despite his protests) willing to leave things like that up to Providence a bit, in case anything might be attempted...?