To attempt to answer this issue, I made a quick review of a book I read thirty years ago:
Mosley, Leonard.
The Battle of Britain - the making of a film (London: Weidenfeld and Nicholson, 1969). ISBN 297 17909 8
The above book emphasizes that Adolf Galland was the primary, if not sole, official ex-Luftwaffe advisor. However, Theo Osterkamp was also involved in the making of this picture, although in what capacity is uncertain. I expect that Galland was the interface between the film makers (Saltzman, Guy Hamilton, Ben Fisz) and the Germans. There are references in the book to Galland checking with veterans on what actually happened to critique this or that perceived excess in the script. There were also disagreements on the basic facts, something which is better known today but an issue that still lingers on in some respects.
Thus one may conclude that the film had one official Luftwaffe veteran advisor but many "hidden" ones unofficially.
Regarding the Stukas...
Yes, the inadequate representations of this unique aircraft type is quite grating (as are the highly wrong engines on the Bf 109s and He 111s!) but the film maker's options were extremely limited at the time. They could hardly build accurate replicas let alone make them dive! By today, there
are some options:
In the mid 1990s I was at an airshow at Elmira/Corning airfield ( later home to the National Warplane Museum) and I set my disbelieving eyes on a half (?) size Stuka. It had some simplifications, particularly in the engine area, but on the whole it was a valiant effort. Incredibly, the fellow who owned and built it was a local guy, Dick Kurzenberger. I looked him up in the phone book and asked him why he built it. He responded that he had been told that no private person could make such a thing and he wagered that that he could! Later I saw and photographed the thing flying along. He also stated that he flew the thing for battle re-enactment societies (presumeably themed WWII and not Civil War!).
Another option now would be to use detailed and accurate radio controlled models. Last year I discovered that a small local firm, Kondor Model Products, makes a beautiful Bf 110 (7 foot wingspan) and a Ju 87B (80 inch wingspan), a Do 335 (84 inch wingspan) among other things. Take a look at
www.kmp.ca
Of course, nowadays computer generated graphics seems to be capable of anything!