Shack and Dingo,
Yeah  it is rewarding, must admit did not expect the result I got, even the garage thought I had gotten brand spankers.
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Will preface this with yellowing against the white haze, the basics are the same just the aggressiveness of the materials used.
Yellowing and light haze
Firstly the exterior, 
Get the cutting compound on the wool pad and filmish pressure in alternating strokes across and up / down. This will get you to clear lens and a pretty good result (good enough for roadworthy!) in virtually minutes and depending on the extent of the yellowing may require additional compound. 
Secondly, The foam pad, medium density, apply the polish on pad and dab on multiple spots on the lens. This will take longer and will require reapplying but the same strokes and the clarity jumps up and out at you.
To do both headlights took around 30 minutes which included looking for the polish. I had the headlights off and had them sitting in a towel on a chair to polish by the way.
Heavy Haze
To do this it is recommended to go the wet and dry sandpaper route, 600 to 1500 to 3000 and a spray bottle, probably easier to take the lens out as its only a couple of screws each side. Soak the paper for about an hour and with soft to firm pad, very moderate pressure sand the lens either across or down with the 600 - I did mine above a half full laundry sink dipping the paper in often. Change to the 1500 and sand against the grain of the 600. Then repeat the process with the 3000 and you will find a pretty good result with regards clarity.
The picture below is very badly hazed headlights which came with the car done this way. It was only really then I found the internal issue of ingress of what looks like coffee!! 
I will note that I am very pleased with the result of the wet and dry sanding as to where the headlights were I got the D2a, they were very weathered and the finished result re clarity after this was much better than the replacement headlights I got by the way and did with the compound and polish you have seen already.
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The internal staining
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The lens from here can be compounded and polished as per the light haze etc.
Re the internal - did not have enough time due to the RWC completion to go through the process of cleaning the inside but the headlight lens can be removed. After extensive searching did find a reference as to the two tabs on the underside of the headlight which can be opened and the lens prized from the casing (reputably the gluing is pathetic so gives away with a bit of effort) but as you can see the original headlights this must have been done before and the clips broken.
 
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To make this easier have seen multiple clips on the net where the headlight is put in the oven at 160-180' for 5 to 7 minutes and WITH GLOVES, the lens will come off easily (the tabs will also be easier to move after the oven and should not break).
Then when the cleaning process on the inside of the lens is done a bead of appropriate sealant is applied and the lens reattached. It may even be easier to clean both sides of the lens when off as the headlight unit is bulky to say the least.
As I have other priorities at the moment with the D2, leaving the final restoration of the original D2a headlights to later.
As I had to change the finishing trim under the headlights over, the removal of the stud at the outer of the trim to the headlight proper - There is a small clip on this and could not remove it, again searched for the easy answer so finished dremeling off the stud and clip, if anyone has done it what is the answer???
Good Luck
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