Hi Marc,
#1. the purpose of number of so called Derived Business Transactions (Update types, usually starting with DBT_XXXX) is to post some non-cash flows or valuation flows resulting from a specific transaction. E.g. let's say your company code currency is EUR, you take a loan of 130 USD, when rate is 1.30 USD to EUR, so local currency amount is EUR 100 and return the same USD amount, however at a new rate is e.g. 1.50, so you local amount is EUR 86.67 - therefore you book EUR 13.33 as an FX profit. this is a very simplified example - it depends also on so called Position management procedure, assigned for that instrument;
#2. I don't think that the G/L account for the Loan position of is an open-item-managed account. Also, there are dedicated transactions in Treasury module, so you don't need to do manual things/ reconciliations in order to calculate and post these Derived flows, e.g. you use TPM1 to execute valuation at the End of Period or TPM18 to post realized gain/ loss.
Kind regards,
Renatas