@eloth:
I decided to make a new thread for this because that one was getting cluttered.
Ok so first of all lets make sure that I understand you correctly. The problem is that when the winged twilight flies (fastforward anim is played) then it slides sideways instead of moving forward or it moves both sideways and forward (I am confused about this from the corpus thread ... which one is it?)
Anyways next we need to figure out whether the skeleton or the animation is causing the problem. My guess is that the problem is with the animation so I will deal with that first.
Here I have uploaded two animations (FastForward only) in which I have applied the hack I had in mind.
First try this one and tell me what you see.
http://www.mediafire.com/download.php?khh3lked7lc56s9
If the twilight flies backwards instead of forwards then use this one:
http://www.mediafire.com/download.php?j7dbq701afyyx0v
Try them and tell me if there is any difference at all.
{note that this is only the FastForward (fly\run forward animation) }
the first one worked, thanks 
can you tell me what you did? or add it in the tutorial?
one more problem, the head and the tailtip isnt animated:

I don't currently have any fix or ideas for the parts that do not animate. As I said in the end of my tutorial .. if they do not animate then they either use morphs or vertex translations and I don't know how to convert them.
Anyways you can apply this method to the corpus creatures. (maybe we can use the winged twilight from the Sovvm project?!)
Here is how I did it.
1) Open the culprit animation (WalkForward \ WalkFastForward) in nifscope.
2) In the block list expand the first entry named "NiTransformInterpolator"
This entry has data for the first\root bone Bip01 which is used for movement.

In it you should find an entry called "NiTransformData".
Click on it and then view the Block Details in the lower panel.
3) You should see an entry named "Translations" , expand it and you should see several "Keys".

4) Now these keys have Translation (movement data) for the three axis (X,Y and Z).
What you want to do is to
- copy the values from the Y-axis to the X-axis
- change the Y-axis values to 0
- change the values of the new X-axis values to negative (-)

Do this with all the values and then save the file.
It is important to understand what we are doing here.
The creature walked sideways (in this instance the Y-axis) instead of forward (in this instance the X-axis) so we copied all the translations from the Y-axis to X-axis.
When we do this however the creature moves backwards instead of forwards ... so we changed the sign of the X-axis translations so that it moves forward.
Just to check your animation, open skeleton.nif
Spells->Animation->attack kf
And select the modified animation file.

Click play and you should see the skeleton move forward (previously it moved sideways)
If it moves backwards instead of forwards then you want to change the sign of the X-axis translations to the opposite sign.
I also noticed that your models have several visible holes, especially the ascended sleeper.
I would recommend turning off autosmooth mesh, weld vertices, and remove illegal faces when you import the model from morrowind.
the winged twilight is fixable i think, the animation works in nifskope, but doesnt in the cs and ingame, but
i noticed that the bones controlling the tail stinger and head, is named something else than the rest of the bones
(eg. head instead of bip01 head), i hope thats the cause.
could also be that those two bones are linked differently than the rest.
yay! i managed to fix the winged twilight
the head, hair and stinger were simply missing
a skininstance 
I will upload it now, its awesome to fight it ingame, its fierce as hell 
Nice 
How did you add the skin instance?
I tried it once with the ogrim and it caused the head to like explode all over the screen.
i added it with within max 8
And selected the nearest bones in the bonegroup 
Alright ... this is great. Theoretically now you can convert 100% of the morrowind creatures.
thats correct 
here is a screen:

here is the download link: