Would certainly be able to get a lot of data if only some small external parts were found by PLAN
That sounds like wishful thinking. Any small fragments of the skin that broke off the jet will have probably moved in a different direction, settling in different areas (if they do at all). They'd also probably corrode unless they could be found quickly. The PLAN could easily spend years trawling the bottom of the sea bed and find nothing of value.
What's going to keep the US Navy top brass up in night is that they will never know how much, if any, Chinese will be able to learn about F-35 from this incident.
That's the same with all espionage. Do you think the PLA is aware of all the information the US or other NATO/5Eyes members hold on Chinese military technology? As much as there is democratic transparency, no one is going to publicise that.
If electronic databases are compromised you may be relatively certain what information was accessed, but that's nowhere near the full extent of what intelligence agencies get up to.
I hope this will make some of them start thinking twice about having the show of FON this close to China.
Per above I'm not convinced there's a real risk of serious damage to the US military from this crash. As you say, China and Chinese nationalists may
want the US to be concerned, but I don't think it will change anything.
Besides, US operations aren't just a "show of force", they provide valuable training opportunities. Operating in South East Asia is not the same as operating off the US coast.