Exactly this. The 800-pound gorilla, causing instability in the region is China, not Japan. Just look at the rhetoric they spout any time anything happens across the whole region. It's not just contested islands though that make up its expansionist agenda, it's claims and naval expansion in the South China sea and its aggressive air defence zone over the East China sea too and its unchecked military expansion. China is the single most unstable,influence.
That's probably exactly why they're so opposed to Japan taking a more active role in the region. They've been able to do as they like and go unchecked for quite awhile. I imagine they're not happy considering any 'peace efforts' will mostly likely be targeted towards the smaller powers China is crowding out.