(Snip of excellent stuff) Would something like this be viable in IYTU?
BEM,
Viable? Hell, yes. Not only viable but designed, written up, and used IMTU.
MTU has all the variations you mentioned; purpose-built IN and IISS models, upgunned merchies, "tar babies" and "honeytraps" to snag borders, all of it. If you surf over to Freelance Traveller, you can read about a Q-ship version of my 300dTon Grote Clan Trader. The Kachina-class is nothing to sneeze at!
Your conjectures about pirates having to board whereas commerce raiders need only shoot is spot on too.
Have fun,
Bill