And you are entitled to your opinion. To answer your question , yes, cross cultural marriage does bring cultural infusion because the marriage of a foreign princess (Bekjae or Tang) means that the Princess will eventually rule as Queen Consort of the Emperor. The Queen will raise her child (the future Emperor) with the mannerisms of her people , understanding of the court concepts from her native country. This eventually influences the future Emperor, who then will influence the Ruling Lords (Daimyos), the Samurai class, the gentry class, the commoners et al. This is part of transformational cultural transmission.
A united East Asia ? Wow, such an interesting scenario. There was a time under the Tang Dynasty when East Asia was truly in harmony with each other. Everyone flourished. In fact it was during this period when Japan's art culture as well as Korea's art culture blossomed in form. Interestingly enough it was during this period that the diplomatic relations with China reached an all time high. If this happened in our history, who is to say that this is not possible in the future? Right?