Gnucash: Importing transactions in multi-currency accounting

I have current accounts in EUR, USD, and GBP, and expenses in multiple currencies as well. The way I track my expenses currently is: if the expense is exclusively in one of the currencies (e.g. UK bills in GBP), then the expense account is in that currency. Otherwise, it’s in the "home" currency of EUR. This requires using exchange rates when, for example, debiting a current account in USD and crediting an expense account in EUR.

I import my bank statements monthly using OFX/CSV. However, after using gnucash for over a year and a half, I just noticed now that when currencies differ gnucash automatically and silently sets the exchange rate at 1! For my aforementioned currency pairs this is an error of ~15% on every such transaction…

Now I’m faced with the following two questions:

• How to ensure that, when importing transactions, the (most recent) exchange rate I set in the price editor is used

• How to edit my previous transactions so that they use the proper exhange rate (short of doing right-click > edit exchange rate > fetch rate > OK, for every single transaction)

If anybody could help that would be great. Alternatively, I’m also welcome to suggestions for better managing multi-currency expenses (though I think my setup is okay :).