Some nice drawing work there. Well done. I was confused by your questions so looked back to page 1.
1). Porsche arrived at 356 because as a design company all those years ago the design number 356 was eventually arrived at and that was the Porsche design number 356. It carried the 356 name then to the end of its design life, with the 356 with designations A,B,C, Carrera for example. I guess design 910 was only changed to 911 because of the French. BUT... who knows the full story this makes the most logical one
2). 13 degrees plus or minus 10 degrees is the nominal value and its tolerance. Every car has differences somewhere on the body,doors, bonnet lids, etc. Its no wonder most repro parts have to be modified somewhat on restoration. The term hand made takes tolarances into account !!
3). Correct, it is the drawing number you see ( Zeichnung )
4). The terms Schlussleuchte and 'Blink' - 'Bremsleuchte' are confusing somewhat. You do not show where the direction line on the drawing goes to.
For example Blink - Bremsleuchte translate as 'Flashing Indicator' - 'Dynamo ( Generator ) light'.
and Schlussleuchte means 'tail light' in English.
Roy