Hi Yannis,
a) There is no option for subset like that but you can load it via custom css file with:
@import url('font_path');
and on frontend, you can call this font as you wish.
For example: https://prnt.sc/rlp6ti
@import url('https://fonts.googleapis.com/css?family=Open+Sans&display=swap&subset=greek');
body {
font-family: 'Open Sans';
font-size: 16px;
}
b) I understand your idea that you don't want to load google font directly like that but at this moment, it can't be deleted, you can add your desired via custom fonts tab and it will replace the current font. The important thing is that if on frontend you don't use google font on any part of your site, it will not be loaded on frontend.
I will also pass to T4 developer for further improvement on this, I think a new option for users to decide to load google font or not will be better.