Issue does not appear to be associated with the attachment (blower) so you can cross that part off you list.
Your shop tech is correct. If you depress the traction pedal and the operator is out of the seat, the unit should shut off. So it sounds to me like the seat switch circuit was altered, but the alteration has failed. At any rate, I would consider this a big safety issue.
I really hate to divert you at this point, but reality is that in order to easily trace the issue out you should have a Service manual, or at least an Operators manual. Also, your local authorized Toro Commercial product Distributor is well versed in fixing GM 455's. You can contact them via their web site: http://www.centuryequip.com/
. This may be the best, safest way to get you up and running.
Toro commercial Service