"watchdog" or "vassal" are terms used by those that either don't know the common security interest between Japan and the US, or intentional just say so because a combined US-Japan force is unbeatable by China. Yeah US and Japan had a war.
Chinese posters here seem eager to want to have a turn in a war with the US. Now that is some baka thinking... Well doing it the slow none-war way is fine and fair competition, but expect it from the Japanese side as well.
Battle of Iwo Jima, well a movie on it, not going to post actual footage of it.
Joint memorial service for the Battle of Iwo Jima
The other parties invite the US and Japan because they need more power backing them in order to strengthen their on negotiation posture with China. Without the US and Japan, then negotiations would be heavily biased towards China. Besides, if China controlled the South China Sea, they could use that control as a from of pressure on trade that goes through and towards Japan. That trade includes oil and natural gas for Japan. So no, Japan will not leave, nor will the US. The South China Sea is international water, respect that by dropping the 9 dash line claim.