..hmmm..
I'm not clear on what you're wanting as a solution to be honest (am new to all of it: osclass, plugins, online payments, etc..) but that aside, the
PayPal Advanced Checkout plugin supposedly handles currency conversions "across borders" on the backend while customers pay with their credit and debit cards (not needing a PayPal account).
If you had this installed, were your questions related to this or were you considering configuring or modifying different payment gateways?
Again, apologies if I misunderstood or coming across as simplistic.