How do people know? For every previous vaccine, myocarditis inflammation has mostly been temporary. If this one is different, how would people know it's lifelong?

It's much, much more likely that reaction to the vaccine is uncovering existing, latent myocarditis than it is causing them. A la https://en.wikipedia.org/wiki/Seattle_windshield_pitting_epi...

Also, nobody who cares about myocarditis seems to be pointing out that you are 16x more likely to get a myocarditis diagnosis from catching Covid than you are for getting vaccinated.

Commonly used vaccines are known to cause widespread temporary myocarditis?

If you search in academic literature from before Covid, you will find all sorts of case studies of myocarditis caused by flu shots:

https://www.onlinejcf.com/article/S1071-9164(18)30491-3/full... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6786665/ https://jitc.bmj.com/content/7/1/53

It seems pretty safe to assume that if you have an autoimmune disorder that causes heart inflammation when attacking a virus, in some people it may respond to the vaccine similarly. (But again, I think the evidence in such cases is that your body would be much more responsive to a covid infection than an mrna vaccine).