Yes you can create Non-leading ledger parallel with your Leading ledger for addressing japan co reporting with diff FYV. I think this is the option because you want to use same controlling area.
But as you now planning to use new GL concept in your JV , and the FYV also diff, It will be always better proposition to implement this as a new instance by org structure for smooth flow of transaction and reports. Here you have also some dependency with your JV company and if you take this instance also in your gr company parameter , may affect your closing activities due to JV transaction.