Two other thoughts on merging with Not for us ideas while I'm here...
First a simple one; don't do it. Rather than merging, mark the new one, not for us. But only if it really is obviously a non-starter.
Alternatively, let it run and reconsider.
I must admit that I don't link not for us ideas when suggesting a merge. Ideas vault ideas, yes but there seems little point in merging with a completely dead idea that may be worth reconsidering.
On the subject of Ideas vault ideas. When a merge takes place, surely these should be re-activated to see if they get enough support for consideration?