To answer the TL note: a fait accompli is basically just something that already happened and can't be undone. If you read a lot of WNs, especially generic isekai or generic "I was the weakest/banished, BUT..."-esque stuff (like this manga), it's actually a somewhat common phrase in those. I've also seen it used in some other manga/WNs that are more 'regular' romcom/young adult romance type works, but anyways...
It's a generic phrase, of course, and can really be used for a ton of situations. But, in terms of manga/LNs/WNs, many authors basically use it exactly how it's used here, as a euphemism for having sex with someone, which of course is almost always a female character that wants to have sex with the MC. In the isekai/追放された works, it's legit this exact scene copy/pasted across dozens of works, where a female character says they are going to "create a fait accompli" with the MC to keep him in their country/party, get the secrets to his power, etc etc.
Thanks for the TL!