The Q0 series and Homeline breakers are NOT interchangable. The Q0T
breakers are tandem Q0's.

No, you are wrong about this. Check the HD website; they have both QO1515
and QOT1515 tandem breakers, for $15 and $24. (Geez, the place I bought
mine was only $17 and I thought that was high). The QO has a clip to attach
to the buss bar and a hook to engage the box. The QOT has a clip for the
box.