Hi Maria,
let’s try to disassemble this… In the G/L, you have only LCY and ACY amounts, named “Amount” for LCY and “Add.-Currency Amount” for ACY. Regordless of what is being posted in NAV, the Transaction Currency and the resulting amount in TC won’t be there.
In Bank, Customer and Vendor Ledger you can have TC code and amount, named “Currency Code” and “Amount” (and derivatives). LCY amounts will be in all subledgers named “Amount (LCY)” (and derivatives). There is another catch: Standard NAV doesn’t write the TC amount into bank entries when the bank account allows multiple currencies (no currency code is set).
So… from a reporting point of view, it is a good idea to use “Amount (LCY)” where available, and “Amount” for the G/L. This will deliver consistent results.
Regarding accounts with different currencies: You can have this in the Bank, Customer and Vendor Ledger. For a bank account, it results in no adjustment available. Adjust Exchange Rates only adjusts the balance for bank accounts, and this must be one currency only from the first posting. The batch filters for currency code in the bank account. For customer and vendor accounts, it can be any mix of currencies. The batch does the adjustment by entry (and by currency of the entry). This is a revaluation on a currency by currency basis.
All in all, the mistake you can make is to fetch the “Amount” fields from the subledgers in Jet and assume it’s LCY. Then you will get probably meaningless data. All other fields should be consistent.
If you think you need more transparency on the currency handling, there’s an AddOn for that 
Here’s a link to the flyer: https://drive.google.com/file/d/0B8eVGp-nWprzLWJleUhZY0gwNXM/view?usp=sharing
with best regards
Jens