I am the third generation in a family which has been working in the textile industry since the Great Depression. I have a college degree in textiles. My father began working in textiles in the early 1960's and is still involved. His father worked in varied textile mills (weaving, knitting) beginning in the 1930's and into the 1970's (he was exempted from wartime service because his supervisor at the knitting mill wrote the war department expressing that he was indispensable to their production and more useful to the war effort in the factory). I not only know how things are produced and made now, I also know how much was done during the wartime textile industry. Is it a totally comprehensive understanding of the processes and procedures of all of the mills at that time? No, but it covers most of the bases from the time fibers begin processing until either yarn or finished cloth leaves the factory. (After this point, I can gain insight from my wife who has a degree in apparel design and knows the workings of the cut and sew industry).
I am not making guesses on what may or may not have happened in the textile factories during the war; I've got a pretty solid grasp on the subject. It is from this perspective I am making my comments on this subject. If my credentials are lacking the necessary approval/credibility, I am sure I know others more qualified who can speak authoritatively on this matter. In the meantime, I hope my observations are of value to the discussion.
Wool is not going to fade due to sunlight with any degree of uniformity. If it is a yarn package (spool), only the outside will fade, and after you remove a few yards of yarn the dyed color remains inside where it was shielded from the sun beneath the faded layer. If it is a roll of cloth, only the outside will fade. If it is a knit cuff, only the side which sunlight shines upon will fade. If it is a stack of knit cuffs and waistbands, only the parts and surfaces in direct sunlight will fade. The other parts are shielded and the resulting fading is not uniform and consistent.
How likely is it that OD dyed wool will just happen to fade to another color used in knits of other jackets? Not likely. It would be like suggesting that UV exposure on modern digital camo ACU's will cause it to fade to the older woodland BDU pattern. Could a strange reactivity of dye make the OD change color to the red? Maybe, but again it is more likely to be changed to some other color, not one used in other military garments. Green is more likely to turn yellow or orange or blue or brown. Green and red are quite distant (opposite) from one another in the color spectrum - it would be like having your blue jeans fade to orange. It can happen, but is not likely. Not likely at all.
Nylon is a different fiber from wool. The chemistry/dyes with which nylon is dyed, and the resulting way it fades is not the same as in wool. So the fading of a nylon flight jacket from the 1950's to 1960's is going to be different from wool. Fading happens, but it is not constant for all fibers and all dyes. Colorfastness is the term used to describe how well a dyed fabric retains its dyed color. Some dyes and some fibers each have different colorfastness. There are plenty of variables: fiber, density of the twisted yarns within the woven or knit cloth, dye type, dye concentration, solution bath chemistry, water hardness, water temperature, batch dyed vs continuous dyed, degree of rinsing - and all of this before the product leaves the factory (either as spooled yarn or as finished cloth) and also before other factors such as UV exposure, or laundering come into play.
If there is some reason an OD woolen knit might change color to red, I speculate it happened in the dye process, long before sunlight exposure is a factor whatsoever. But again, I think this is a long shot and not realistically plausible.
More likely, in the case of these B-10 jackets, The best possibility was the use of red knits already in existence. The sun- fading and reactive dye hypotheses are inconsistent with observed realities of the materials and manufacturing processes involved.
We know this was a rare occurrence. I am curious if the original red knit B-10 had shorter cuffs, as the B-10 knit cuff is longer than the A-2. The length of the cuff would be a strong indication of if the surplus A-2 cuffs were used, if it is the shorter length.