The architect was David Ewart (1841-1921) see: http://dictionaryofarchitectsincanada.org/architects/view/1559 for a list of his great works. Very prolific!

I'm happy (and sorry) to say I did find his death record at ancestry.ca as well as his interment record from Beechwood Cemetery (record #18674) which lists his occupation as architect. He didn't die from a fall/jump from the roof of the museum, it was cancer of the stomach that took him at age 81. Which I guess doesn't preclude him from haunting the museum, but it does take some of the fun out of the story. I'm attaching a pdf of both records for your review.

Thanks for the great site, I'm really enjoying reading about all the hauntings around Ontario. I live in Manotick and have heard the story of the ghost at The Mill for 40 years, I've been there a few times and have passed by it hundreds of times with no experiences or sightings (and I do look every time!), but my kids tell me they have seen things - and now that I know about the 1:00 - 1:15 time frame I'll be heading back soon!