Hello,
1) You can keep these options disabled (actually we do not enable them on our demos).
2) You can set Default in the Elementor settings https://prnt.sc/26kx6vc and it will inherit theme settings. If you choose anything there then Elementor elements that you will add to page will use settings that you set in the Elementor settings.
3) Add the below code to Theme Options > Custom CSS to change the default site header font-size
.site-header {
font-size: 16px;
}
All the other header font-sizes will be calculated depending on this.
Regards