Hi, I am more then a beginner so my observations may be way off ,and I'm sure some of the more knowledgeable members will help correct me . That being said, the mei does look off. Of the Sukenaga's I found most signed with a different version of naga. The one I found who used this one is suk347 who's style, as you already stated, doesn't match. Also I didn't see any examples of these smiths signing so high (above mekugi-ana). The sugata does match the late nambokucho period. Also from what could be seen and what you stated such as the masame and the activity in the hamon I believe it could be Yamato influenced. All in all to me it looks like it has some serious potential. Definitely wouldn't mind adding it to my collection. Best of luck with it.