To clarify - I'm only for it, and prefer using Debian tools. OBS should support both RPM and DEB really. If you want to propose that to Mer developers - try catching them on IRC and share back their response here.

I don't think Mer is compatible out of the box with Fermantle structure, because besides packaging there are several other architectural differences. At this point Mer is quite far ahead so switching back won't be easy. But if you have convincing reasons and help - propose it.