1, 11 and 18 for EpelYuri!! :)
Thank you for the question! <3
1) It depends on the side as they have pretty different feelings about each other.
From Epel's point of view it's my strange addiction by Billie Eilish or Crush by Tessa Violet. He fell in love second and at first it was hard for him to understand why but it was an intense feeling.
From Yuri's point of view it's Don't you give up by Dansu or Селфхарм by Монеточка. Both songs are about feelings of insecurity and that's what Yuri feels, even though he knows that Epel is not the type to just abandon him, he has that irrational fear. For him the boy feels like "like care, like home, like the ability to love seriously and endlessly".
The closest to their dynamic would be Олимпос by алёна швец. It's about young reckless love which is just perfect for them.
I smell like rock-n-roll,
"Where will you bring me today?"
I will bathe in a fountain.
I breathe out smoke rings right in your face:
This whole town will soon be ours.
The sun shines over you, the alcohol pours
If you ever get in a fight, then I'll go with you
I will hold your golden locks, if you drink too much
I will be there with during the last lesson, if you call me
11) Yes, they do! It's mostly Epel that uses them, but Yuri also has some. They both think it's kind of cringe but they sometimes call each other or think of each other using those names.
The basic ones, that are just another version of their names: Yura for Yuri, because It's actually Yuriy and that's very formal, so when they become friends they drop the formality and Yuri allows Epel to call him in a friendly way. The same goes for Epel, it becomes Epy or Ep. It's not actually informal but it does shorten the name. They say it often.
More private ones for Yuri are: rakas, rakkaani, kokhanyy, durik, kartoplya. Th first ones are just variations of "love" and "dear". The last two are memey nicknames. Durik is durak(fool, idiot) + Yurik(cute way to say Yuri). And Kartoplya is literally just potato, the iconic nickname from Vil.
And for Epel: bakango(baka+ringo, idiot apple), pridurok(idiot), moy(mine), yablochko(cute for apple), solnishko(cute for sun), kartoshik(small potat).
I headcanon that Epel is Finnish + Slavic, so he knows some wild mix of Russian and Ukrainian. So they understand each other when it's something Slavic and cringe a lot, ahah.
18) Epel didn't really want to get too attached to someone from another world, so when the realization of his infatuation hit him like a truck he sweated bullets. He postponed the talk for long enough until they had a fight somewhere at chapter 6 and Yuri confessed that he isn't coming back in any way to his world. Not only because he felt like twisted wonderland was his home now, but also because Yuri is dead in his world. He just can't go back. So now Epel is more concerned how Yuri will live after the college as he has no legal documents lmao.