I wouldn't read too much into these percentages. If 70% mainlanders are willing to fight against an American or Japanese invasion, somehow that will increase China's chance of winning? it will only get more people killed. PLA increased its fighting power by cutting personnel.
At this day and age, any talk of a military conflict across the strait with the PLA losing is pretty delusional, with or without US involvement. the only matter is cost. Sure Taipei can find ways to increase PLA's cost. But any additional cost on the PLA, will have a price on Taiwan as well. There is no scenario in a military conflict Taiwan will come out on top. I am a mainlander. But I treat mailanders or Taiwanese exactly the same. The reality is the mainland is big and Taiwan is small. So if the Mainland decide to use military force to solve the Taiwan issue, then Taiwan is screwed, no matter what.
To me, the cross strait issue is purely political, not military. Why? All Beijing/mainland need is a name change. Thus no leader will put lives on the line just to gain a name. And few Taiwanese will actually think changing from ROC Taiwan to PRC, Taiwan will make any meaningful difference and worth dying for.
That is why DPP is brilliant. They don't really talk about independence. Instead they focus on democracy, as if Taiwan becomes part of PRC it will be forced to adopt a different political or economical system. If the people of Taiwan can't recognize this lie, and thus become less willing to compromise with Beijing, then they will unfortunately end up with less autonomy.
If there is any wisdom in Taiwan, people should recognize now if the best time bargain with Xi to get the most out of reunification for Taiwan. Only a powerful leader like Xi on the mainland is in the position to give more.