I've been sitting on this since late December 2012. It appears, according to this email I received, that a lot of the unused stuff came from the Japanese version of the game (please forgive the grammar and such. English is not the first language of the person who sent this and he was using some kind of translator to help):
 
First, unused enemy Graphics(except Sheltem) was really used in the FC Version. The top image for the priest and Druid, the second image for Ghoul and Rotting Corpse, the third image for Hag. (I do not know why they had to be changed.)
You know, I always did think the enemy sprites used for the Priest/Druid/Cleric, etc. and for the Ghoul/Ghost/Rotting Corpse/etc. were reused an awful lot - even for this game.

The last image appear when defeated an enemy with arrow. This came to perform animation by changing a color palette.
This is in reference to the concentric rings. Killing an enemy with an arrow in the US version just causes them to die with the standard "death by weapon" animation.

 Probably Unused Player Character Profile Graphics is connected with a manual for Japanese release. In FC version, initial member of the inn had (e.g Millard, Gideon, Adelaide) the name totally different from the NES version. And the detailed setting about them was written to the manual. Probably, the early plan, would be to let a game more strongly reflect the personality of the character.
Without actually having the Japanese manual, it's kind of hard to follow this exactly, but it sounds like what he's saying is that the original plan of the Japanese release was to have preset characters with personalities, names, and background stories, and the unused drawings would have been their in-game profile pictures. That would make sense, because if you count them all up, plus add the Cleric of the South which I suspect was also meant to be a party member avatar, there's exactly six of them - one of each possible member of your full party.

 About an image of the Lord kilbarn, this originates in the text of the Japanese version. A text same as a PC version is displayed with the NES version, but with the Japanese version the Lord kilbarn is to die in this event. Because when a party came, the Lord Kilbarn is already attacked by a monster and is critical condition, this image shows that death is coming off on Kilbarn.
Weird. That definitely explains that, then. It just seems like it would also have fit very well with unmasking the fake King Alamar.

All images of Unused Cuscenes are used in FC. The first one was called "a sword of old God(ふるきかみのかたな)". It was a strong magic weapon and was placed in Sorpigal 15-5.
The second is the mayor of the town of Portsmith who is the temporary figure of the succubus queen. The mayor is at the spot where the succubus queen appears and can meet anytime until uncovers the secret of the town.
Ah, dang, so he wasn't the Data Keeper or a Judge after all. Well, at least we can stop arguing over it. Emotgiggle

The image of the tree in the third was a thing for big trees called "Golden Baum" located in Area C1 7-7. This tree existed as a mark to obtain a flute ruby whistle. It is thought that these were cut to bring a game close to the specifications of the PC version more.
Yeah, there's definitely no tree for that event in the US version. It just shows the treasure chest cutscene when you find the Ruby Whistle like for any other treasure you find. And I'm starting to agree with the last sentence there. It kind of seems like they spiced the game up a bit for the Japanese release, but somebody wanted the US version to more closely resemble the original US DOS game.

The owl appeared in several places with the FC version, and gave the party a hint. It is a soul of King Alamar, this is explained based on Japanese version original setting that a crest of Alamar is an owl.
Sort of like Raura/Kaeopora Gaebora...

"High court sentence: years" This text is translation of used with a Japanese version. I didn't know that NES version did not have an arrest event.
So, you can actually get sent to jail in the FC version. I still wonder how that's gotta work.

The text displayed in the beginning of the FC version is the following meaning... (according to the machine translation) "This is a story of the origin. The times when a called the sky the heavens and called the ground the earth. Distance of the far-off ocean, the world place that are over. There was the continent which the might and magic ruled over." Interestingly, there is an unused kanji called "英雄伝説"(It mean "the aretalogy") in character string left in NES version. This shows the possibility that the text for the another plot surface was cut.
This sounds vaguely similar to the text on the Dragon Plaque in the US version. That might be why that plaque was added to the US version since the opening story was removed.

The text of plaque is totally different between NES version and FC version. plaque for FC version is called a "relief" and talks using telepathy.
Well, that's just bizarre. It just seems to make more sense for a relief to have writing on it that you read rather than for it to talk to you via telepathy.