Took a bit of digging, it's the giant flying dragon he resurrect after getting the dungeon:
https://mangadex.org/chapter/ba0bf822-4452-4d9b-a583-f9f6ac96feac/14
Mikoto is too pure to think of sex, and Takase is too cowardly to actually go for it.
Clearly it'll take Takase's mom being in the room and telling them how to seggs before those two do anything.
The translation is a bit derp about it, but to put it properly
He was declared crown prince at 7.
There's a period of the king/queen looking for his fiancee that wasn't specified.
But doing a little math show it's 2 years: she's 4 years younger than him, she's 5, therefore he's 9
And...
Because it wasn't bandits but knights/soldiers in disguise.
From what's shown, the deciding factor is simply FL's offer to welcome them in her country.
In the OG she probably wasn't this determined due to not having the alternative option FL set out for her here.
Even if she fought back, won...
I the little text on page 21 is just 'refers to vol 3'
Specifically, this particular page from ch 15: https://mangadex.org/chapter/338778ee-de8a-42db-b3f6-673e8fb4b266/4
Despite it not at all fitting if I really think about it, my first thought:
"what if Ruruka is an even bigger bottom with trauma and Suuna an even bigger top"
She knows he knows flower language.
It just took him a bit to realize she's been 'speaking flowers' because it's not his primary language + she probably hasn't tried this before.
My personal theory is it's his parents who suggested this idea to her.