Seems to me like you're making things a bit complicated when mirroring...

The way I do it (and probably anyone else who's
ever bought one of Kurt's [GREAT] tutorials ) is I select my smooth proxy (or whatever), Duplicate it with Instance option turned on, and put in the scale of -1 in the axis i want it to be mirrored over... Just just have to make sure that the object's pivot is at the edge where you want to merge.

When the model is complete, just delete the history on the instance, combine the two parts, and merge the vertices down the middle...

Hope this makes sense