Lithuania's boss tells him that he will marry his neighbour, Poland (and their bosses will get married, too). Thus, a group made up of Lithuania and some of his men travels to Poland. This is how they first meet.
...and there our problem is. Uhm. Anyone who has followed our muses at least the tiniest bit probably understands what it is. The author just made the marriage be about 200 years prior to what the fandom had decided.
The solution? Sit and listen in awe!( cut for tl;dr )
So, the only major thing that I'd need to retcon would be that they were engaged politically in the future. Let's just say that never happened, please?
Also: we need everyone to kind of go "la la la, that didn't happen" to any reference to them knowing each other before Jadwiga and Jogaila's wedding. And say that instead of "ahhh, I can't tell him, he'll hate me forever and no one will ever approve!", the whole thing they've had lately has been "ahhh, I can't tell him, this marriage thing is supposed to be chaste and not personal, it's just politics and there's no way he really likes me like that ><;"
Aaaalso. Their engagement ceremony? It's a ceremony to reaffirm already existing bonds, first stage.