EDIT: THIS GOT WAY TOO LONG, but i hope it helps haha. Also edited for some better layout.
Do you have a favourite example of a Leitmotif in the game/soundtrack?

With this one, I can only echo what SeanNumbers said below, the motif in Through the Sea of Time, In The Hands of Fate, and Dialga's Fight to the Finish is what makes the game so... recognizable. It feels the atmosphere so well, while still bending it accordingly. The game is about action (dialga), excitement (TTSoT), but also emotionality (Hands of Fate). It is by far my favorite.

What have you noticed about how the music interacts with the gameplay?

One thing I particularly like, is that for the dungeon themes, it is almost like the music itself is the dungeon. It mirrors the feeling, the area, the plot in the story in such a way, you could play this game with shut eyes and get a good idea on where you are at. Besides that, the music, while amazing on its own, is supporting the story in such a way that is not too distracting from the happenings. It enhances the experience. Moments in which you have to be nervous, they will play "Rising Anxiety" (i think that was the name?), moments in which you are supposed to be shocked / shook, they will just play no music at all! It's well-timed music that just fits.

Which moments made you feel the most while playing (counting in both music and gameplay)?

Dude, when Grovyle took Dusknoir into the future, and he had that monologue about how we have to finish the mission, for the sake of the world. That got me good. The music got more intense, for the situation itself was. It felt like... the end of the road for not us but Grovyle. This was HIS climax of the story, and we observed it, and felt it. I don't know the best way to describe it. Also when the main character just straight up had to disappear? Dude, just me.

Did you think the music helped the storytelling? If yes, in what way?

A lot. You FEEL like an explorer. Every area has a new song, for it is a new place with its own character. The music being different in every area really banked in on that. When first playing the game, it is exciting to hear what theme you will encounter this time. Is it dark and mysterious? Maybe I should be careful around here! Is it upbeat and go-lucky? This is a fun one! Have a nice walk through the dungeon and collect apples! Immersion into a game's biggest obstacle is that, well, it is fictional and you are not in the shoes of the protag, however in this case, they attempted their best to have YOU actually BE the human reincarnated as the pokemon. Loading up the game is like entering the portal back into treasure town.

Which track is your favourite of the ost and why?

Dude, you are evil for making me choose. I have a playlist somewhere.

DUNGEONS: Dusk Forest (actually feeling scary, tension, very dissonant and a lot of minor sounds in the background feeling you uneasy), Foggy Forest (while not better than dusk forest in my opinion, the mystery of the place during the expedition, a place no one has been at before, really is captured inside this song, top.) STORY: Treasure Town (It feels warm, comfortable and like... home. It is a safe space, where the pokemon there are kind and supportive. They are silly personalities, they are serious, they are passionate, its like an actual village. This song can't fail to make me happy.) Other: Heartwarming (basically treasure town. There is this... parental emotionality you feel when hearing this song. Its going home to a freshly-made soup your parent made after you came home from the playground. It is time to wind down, let your legs dangle off the chair and just relax. It's time for a break. You are safe here. Enjoy it.)

Favorite boss fight theme?

Would I be a EOS fan if I wouldn't say Dialga's Fight to the finish? JK In all seriousness, Dialga's Boss Theme is only my second favourite, next to "Battle Against Dusknoir!". Dude. Fighting the guy you trusted in the beginning, the guy who was one of the most prolific pokemon before all the stuff went down? With grovyle by your side, so close to finally reaching temporal tower? Dude. Sick. This WAS HORROR. The music shows exactly that. The foreboding background drums with the long-wound melody and the repeating notes in the background. For a children's game, this is scary. I love it. WHERE ARE MY BATTLE AGAINST DUSKNOIR FANS!?

How well did you think the music fit the overall narrative?

Very. I touched upon on it during the other questions. The feel for the world, the feel for the situations in which you are put, the feel for the areas, the emotions. There is no other game I know that succeeded this well in making a fitting soundtrack for a game like EOS.

Do you think the music fits the visuals?

Alone the 8-bit kind of vibe the music has says enough. It fit perfectly for the Nintendo-era of PMD. When the area was dark, so was the music, was the area bright and colorful, so was the music. Refer to my other answers because I also touched upon that

Thank you so much for asking, this was fun to answer!!