RE: Shall i create a new model for Japan like "JAP" MODEL for development
I think you should discuss with your team how the code delivery process has been until now. I'm almost 100% sure that there already is some process, which you can use as it is, or adjust it. Could you please describe the current code delivery process for this customer? Until that, I can't provide any additional suggestions (except the ones in this message).
But yes, you are right that in my process you deliver all customizations to test system. That's how you should normally do: deliver a model with all your customizations, not only the latest changes.
For example, let's assume that your test system already has customizations A, B and C.
Now you develop customization D in your dev system. Your model delivery will contain customizations A, B, C and D.
If you don't want to deliver everything from your dev system, you need another system for collecting the delivery contents (in my description I called it "Delivery system".). Then you put there all the code that you want to deploy, export model from that system and import it to test system.
You can check Microsoft's whitepaper "Deploying customizations across AX2012 environments": www.microsoft.com/.../confirmation.aspx