First, you might want to switch your NIV and KJV references around in the first part of your post....you have them backwards....smile. (edit: never mind, you switched them!)
Second, The ONLY time the KJV uses the term "Lucifer" is in this verse. It is Strong's listing 1966, and the word is "heylel", which means brightness, or the morning star and is TRANSLATED by the KJV as Lucifer. This is the only time this word is used in the entire Bible.
I think the reason the word "heylel" was used can be traced first to Ezekiel 28:12 and on, where God is addressing Satan via Ezekiel and then through the King of Tyre. There we see Satan, or Lucifer (which means 'light bearer') was anointed the guardian cherub of Eden. His name meant 'light bearer' and he was there as part of the first part of creation when "the morning stars sang together and all the angels shouted for joy" (Job 38:7).
But there is something interesting to note here. There are only FOUR times when 'morning star' is mentioned in the Bible. The first is in the Isaiah verse under question.
The second is in 2 Peter 1:19
NIV: "And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place until the day dawn and the morning star rises in your hearts."
The KJV says 'day star' there. But the word in the Greek is 'phosphorus', and that was the name for Venus when it was the morning star (vs. the times of the year when it was the evening star, or "hesperus").
Hold on to any ideas about the meaning of that....
You know the Revelation 22 reference, where Jesus says HE is the bright and morning star.
Now go to the one other place where this term is used:
Rev. 2:26-28 (Jesus is speaking to the church at Thyatira):
(NIV) To him who overcomes and does my will to the end I will give authority over the nations --
'He will rule them with an iron scepter;
he will dash them to peices like pottery --'
just as I have received authority from my Father. I will also give him the morning star.
in Rev. 2:28 the term is "proinos aster" or, quite literally "morning star" in a very physical sense.
In Rev. 22:16, we find "orthrinos aster" -- orthrinos means 'relating to the dawn' but also relates to Venus! What is interesting is that the word Orion also comes from 'orthrinos', which refers to the 'bursting forth of light.'
In every case the 'morning star' terminology refers primarily to light.
Lucifer, the 'light bearer' was there in the morning of creation, and as such his very name is related to that fact and the use of Isaiah of the term that means morning star would hearken back to creation week and NOT to anything of Christ. Especially since, in all the prophecies of Christ, that term is never used.
When Peter uses the term, he uses a different connotation and a different sort of word (not just a different word), in referring to Venus, as the last light before the sun comes up. This fits the usage as he is talking about prophecy until Christ, who is the light of the world.
The first time Jesus uses what we translate as 'morning star' in Revelation 2, it is a promise to the church that this is something He will give them, along with authority. For me, the meaning here is not clear, but I could ask, "Does that mean symbol of authority?"
Lastly, when Jesus uses the term in relation to Himself, He uses the term which means a light bursting forth, not gradually rising from the horizon!
English is, in some ways, a poverty-stricken language, and as such can cause confusion such as the lead post here indicates.
In Isaiah, the KJV is right in identifying the person being talked to.
The NIV is right in identifying the way he is being spoken to.
The Hebrew indicated both of these at the same time, but English can't.
I hope that helps a little.