Hi,
While I do not fully understand your question, still let me try to guess.
" You are looking for a fool proof method to say a particular GL Account in Tax Data Documents is belonging to a Customer or a Vendor."
If this is the case, then here are some pointers I can give regarding your queries:
Query # 1) BSET-KTOSL:
While the fields you mentioned in your Query # 1 - BSET-KTOSL with the respective account keys are fine, you can add the field Tax Code - MWSKL as an additional criteria to make it fool proof. Tax Codes indicate if it is a Output[Read Customer] OR Input [Read Vendor] Tax.
Further, in the Tax Code Customizing Table - T007A - Tax Keys, there is a field Called Tax Type[MWART] which indicates whether this is a Input / Output Tax Type. This is directly pointing to whether it is related to Customer[Output] and Vendor[Input].
Query # 2) BSET-KNUMH:
You could technically use it but having Tax Codes would make it more robust.
Hope this helps.
Thanks.