Hi Jens,
A - All Key Figures in S&OP are stored in type Decimal (18,6) . This is sufficient for all Value and Qty KFs. The KF fields (if KF is stored) are generated and stored in HANA tables upon activation.
You cannot change setting to a different length and this is not required. If you have only 4 digits, the data is stored for only 4 digits.
B- same as A.
The decimal separator is period only.
Hope this helps.
Thanks,
Raghav