Right on both counts. "Porte" means "door", "port" means "to carry/wear". The latter is the word used in "portmanteau". I also agree "portmantout" sounds nice, and very similar to "portmanteau".
Thanks for the polite correction. (I said 'porte' when dissecting the word in my original post, even though I did get the word 'portmanteau' as a whole correct.)
Furthermore, nowhere does the idea of porte-manteau carry any meaning as to the exercice of the original post which looks for partially-overlapping words.
Edit:
That last paragraph of mine is just wrong. I just read Wikipedia and indeed the English word actually contains the ideas of overlap/truncation. That comes from the redefining of portemanteau (as a coat hanger, or the original term of the person carrying a coat) into a suitcase that splits in two equal parts. I just always assumed it described the way porte and manteau were stuck together (in which there's no overlap/truncation), instead of the suitcase meaning.
* I still maintain there's just no reason for the English word to have been bastardized into portmanteau.
* Also the fact that portmanteau/portemanteau is used to describe words being combined in a different way to how porte and manteau are combined just seems needlessly confusing.