I further clarified this question with my father-in-law this morning. Starting about 35 years ago, during Taiwan's economic boom, the majority of Taiwanese fortunes were invested in mainland china, mostly in manufacturing (like the rest of the world). This peaked around 2000. By that time three things happened;

1). Chinese companies had "learned" (aka stolen) enough processes and technology from Taiwanese firms that Taiwanese firms were no longer competitive and most families left China and retired.

2). Local authorities essentially pushed Taiwanese owners out and or took over their businesses, often when it was something more complicated that they couldn't steal fast enough.

3). Those Taiwanese that are still invested in China essentially became Chinese and renounced Taiwan all together.

Now-a-days the majority of Taiwanese investment is in Southeast Asia, as I said, mostly Thailand, Vietnam and Malaysia. However, there is a significant more amount of investment of Taiwanese money in Japan.

Japan and Taiwan are extremely friendly with one another. Taiwanese generally love Japan, especially those under 40 or so (the older generations have a more guarded view of Japan due to the occupation during WW2). Many Taiwanese speak Japanese and doing business with Japan is generally very easy for Taiwanese (i.e. both countries have the same dependence on paperwork and bureaucracy, plus the banking system work in similar was/rules). Plus, due to the economy, Japan is really cheap right now to invest in.

Lastly, I confirmed that Mainland Chinese are forbidden to own businesses in Taiwan. This has been the case essentially from 1949 onward, with a few exceptions. Mainland Chinese can own residential property here, but it is very difficult from a Visa standpoint (PRC citizens need very special Visa to leave China and additionally enter Taiwan, both China and Taiwan need to issue them, they are two distinct separate counties after all) as well as the repercussions a Chinese citizen would face in mainland China if hey owned Taiwanese property.

There are a few political parties in Taiwan, which has double digit political parties, that are pro China, pro-unification and pro-PRC, but they get smaller and small every election cycle, mostly due to their insane rhetoric and their members dying (average age for this parties id 75+).