Are these three fears justified or should I just take the plunge?
You should take the plunge. I have a 2010 MacBook Pro 15 that was stuck on Snow Leopard because it was a work machine. I was finally able to upgrade it directly from Snow Leopard to Mountain Lion and it works fine. The only issue I had was my work-mandated antivirus, Symantec Endpoint. Apparently the antivirus companies who stay on top of the latest zero-day threats to your Mac had no idea Apple had released a new operating sytem or had even gone to a 64-bit architecture. Review what 3rd party softwrae you have installed and make sure it is compatible. There is a lot of software from 2002 that works fine in Snow Leopard that won't work in Mountain Lion.