The answer is, unfortunately: You can't. The Outlook command line switches do not support attaching multiple files.
If you really want to use this approach to starting outlook, zip your attachments into a single file, then attach that.
If you need user interactivity when sending the email, I'd suggest revisiting the COM approach. I know you've discarded that, but I'm not convinced your technical issue cannot be solved (hint: start a stack overflow question).
If you do not need user interactivity, by all means use SMTP. Firing up a new outlook instance for each message tends to be rather fragile, as one hanging instance can stop all subsequent instances from appearing.