Because Datawatch doesn't support Monarch 10.5 on Windows 8.1 doesn't mean it won't run 8.1. There is little to no difference in how programs run on Windows Vista,7 or 8.x. It if runs on one it runs on them all. I seem to remember Datawatch stating that 10.5 was not supported on Windows 7 and I have been doing just that for years.
Tread lightly making the jump from 10.5 to 12.x. It is a totally new program with totally different ways in doing things. I have been using it for over a year and still fall back to a PC with 10.5 installed when I need to get work done.