Have you tried contacting them directly about this issue? If they confirm it is an issue for matchmaking have you tried asking if they'll change your PSN accounts region to JPN?
If there actually is a matchmaking problem, it will be on a game-by-game basis, as some games will use there own matchmaking systems, rocket league/fortnite for example.
And yes DLC for games only works if purchased on an account with the same region as the game.
I'm pretty sure you can't change your region, it's an usual issue with PSN. And changing the region would surely affect my entire games library...
Sure, some games will allow players to change the setting themselves, but not all of them.
My issue is not even about DLC but online currency. They let me buy it from the game on my JP account, but I can't use it, and I can't even get a refund. It's ridiculous, they shouldn't allow the purchase in that case.