You have some superb paperwork, lepingouinmecanique and have provided answers to questions 5 and 6.
That it was advertised as "Allwin de Luxe" and not "Allwin Reserve", with no special attention drawn to the reserve feature, implies that the Allwin de Luxe was a reserve game as standard.
So, with contributions from Germany, France and Australia, we seem to have solved all but the final mystery:
(1) The vast majority of Saxony allwins were made by Jentzsch & Meerz (2) but because the game's large element of chance did not sit well with German authorities and players (3) they were made almost exclusively for export to Britain (4) where a more amusement machine-tolerant environment provided a good market, and comparatively low German wages made them price-competitive. Wars, laws and economic turmoil also played their part. (5) They were reserve ball games, (6) originally manufactured as token payout and later also as coin payout versions.
I thought mystery 7 would be unravelled first. Like Gameswat, I'm sure I discovered the purpose of the little gate when restoring one - and, like Gameswat, all I now recall is "it was there to stop you playing a ball until you'd done something first". I've seen many allwin reserves, but most are missing this part, so it must have caused trouble.
Of course, new questions come to mind reading this thread:
8. Who invented the allwin?
9. Who made the first version with an open track (a big improvement on the closed spiral of the earliest games).
10. Did Jentzsch & Meerz invent the reserve ball feature or was it found on earlier non-allwin-type games? Date? Patent?