The whole black/white lodge was going somewhere else, as anyone who has read the script for the last episode would know.

So, I read the script for the last episode...true dat, it was goin' somewhere else!

I'm not sure how much it matters to the end product, though. Don't you think Lynch shot it how he wanted to?

The end product is definitely much better than the script. However, the original ideas of the script - which I don't think Lynch had any real say in (though I could be wrong) - are largely unchanged. Coop enters the Lodge to save Annie, offers his soul to save her, then loses some of his courage, and pays the price. Also, the deal about Glastonbury Grove of the last episode is pretty nifty, and that was in the script. The main change, apart from toning down Windom's over-the-top character in the script, is the fact that it's in the Red Room. I think I'm leaning towards the idea that the Lodges (both Black and White) look the same to Laura, Leland and Dale, but might look different to another person, like Jeffries or Briggs. (Or it might just be that the White Lodge was supposed to be made of marble (to everyone) and have a throne for Briggsy until Lynch changed it, last-minute-style.)

It's definitely retcon. But good retcon.

"What credit card do you want to put that on?"
"Caash, prease."

When Coop investigates Dead Dog farm (ep19) the real estate agent tells him of the legend of the farm: "Of all the people in the world the best and worst are drawn to Dead Dog.. most turn away. Only the purest of heart can feel its pain. And somewhere in between, the rest of us struggle." This ties in with the other local legend (the one that Hawk tells Coop about in ep18): "My people believe that the White Lodge is a place where the spirits that rule man and nature here reside. There is also a legend of a place called the Black Lodge ... the shadow-self of the White Lodge. The legend says that every spirit must pass through there on the way to perfection. There, you will meet your own shadow-self... But it is said, if you confront the Black Lodge with imperfect courage, it will utterly annihilate your soul."

Together, these legends suggest the idea of the progression of the soul. Somewhere in the middle, most of us struggle, but probably through reincarnation, we have the chance to gradually increase the purity of our hearts (which could refer to pure good or evil). When one's heart is sufficiently pure, the BL is entered - if it is faced with perfect courage the soul may pass to perfection. If not, the soul is annihilated. Major Briggs and Earle have both been attracted to TP, though the purity in each of their hearts is very different.Major Briggs is being used by the WL spirits in an attempt to infiltrate the BL. They require someone exceptional, and with perfect courage so that they can enter without being annihilated. From time to time, Major Briggs is abducted by the WL Spirits. This is what happens when he disappears from the campsite with Coop (ep17). It has also happened many times before (as revealed by his wife in ep18). It seems that the Major is being sent into the BL, probably on a kind of fact-finding mission. When he returns, the WL spirits take his experiences from him to learn how they may enter the BL themselves -- leaving Major Briggs with no memory of what has happened.

I don't think the Giant comes from the WL.He's a Black Lodge inhabitant,such as LMFAP,BOB,Mike/Gerard are(and probably the Tremonds/Chalfonts)! LMFAP also helps Cooper,giving him some very important clues,but that doesn't mean he's from the WL.Mike/Gerard tells Cooper that the giant is familiar to them,so that's why I think he comes from the BL and also cares about the garmonbozia thing(like the rest BL spirits) ,and perhaps his intentions to help Cooper are not so pure,either..

As for the White horse in Sarah's visions...I think it just represents death(as far as I know white horse is a symbol of death) or it could symbolize drug use...We know that Leland/BOB drugs Sarah every time he wants to do his dirty job;)
 

Well, it seems that Mike was originally from the BL, and was a killer, but then he saw the face of God and was purified, effectively "changing sides" from the BL to the WL, and when Cooper first encounters Mike, Mike is on the side of the WL, as it were.

In The Book of Revelation, a white (or pale) horse is indeed a symbol of death, as you say. "And I saw a pale horse, and its rider's name was Death, and Hades followed close behind him." Rev 6:7-8
 

Mike says that BOB was his familiar, so BOB may originally have been Mike's lowly servant. It was BOB's job to collect fear for Mike in the BL. But BOB began to keep some of the fear for himself, and by feeding off the fear that he acquired he became very powerful -- powerful enough to inhabit humans outside of the BL. For a while, Mike enjoyed this, and the two became partners in killing, finding 'rich pickings' of fear in the human world and all the while increasing in power.At first BOB still needed Mike to gain access to the human world. When someone read the FWWM tattoo on Gerard's arm, Mike would have the opportunity to move into the WL. Once there, Mike could invite BOB by reciting the chant himself. However, BOB's power continued to increase, and when he found Laura, who liked to play with fire, he no longer needed Mike, and began killing on his own so as to keep the fear for himself. Mike became fearful of the power BOB gained and realised that BOB would have to be stopped before his power exceeded Mike's own. Not knowing that BOB had an alternate route into the WL (Laura), Mike cut off the tattooed arm so that BOB would not have the opportunity to enter via Gerard.When BOB kills Laura Palmer he leaves the phrase FWWM written on a piece of paper so that he will be able to return when the note is found and people read it. Mike never did see the face of God, that is just a cover story to enlist the help of Coop and the others in finding BOB.

The Giant goes to a great deal of trouble to give Coop the impression that he is trustworthy and helpful ("I will tell you three things..."). He also wants to find BOB, being threatened by BOB's power. The two of them are using Coop and others to help track down BOB so that they can stop him.