Hi Sebastian,
now the 2nd part. I am a little bit confused, why you are referring to a transferaccount and how you would like to achieve the clearing on customer accounts.
Mainly you have the TradeReceivablesPayablesAccount (Customer Account / Supplier Account) with the Invoices, credit/ debit memos and the payments for the account. With the ManageJournalEntryIn you create Accounting Notifications, which can be an open item. Now you have the payment for 20 invoices - you still need to split and repost it.
So here again I need to understand, what do you mean here. If you like write the comment in German I try to answer in English then.
Best regards
Marlene