it wasn't explicitly said but it's quite obvious what happened to him.
him getting destroyed bc j surpassed him is heavily implied
also the characters mention the whole name thing often in the story
and you should keep in mind that m and j are a parallel to w and j and m is looking for someone to keep the cycle
No W was destroyed to protect J. J lusted after him and as previously said werewolf skin (i believe) is poisonous and a common commodity. J wanted to touch W so bad, that W agreed to dance with him. But I'm assuming that when J said his "innocent lust" is what accidentally killed W, he meant that Wald said his name to avoid seeing his son die. While Werewolves are demons its important to remember they had their own little contingencies like their skin too. So, in the end, Wald died denying Jonathan's lustful feelings for the of familiarity, and the only way for J to die atp was to be forgotten or have mako surpass him. J never loved Mako cause he was deadset on Wald, perpetuating the cycle of love not being reciprocated since M loved J and was jealous, I believe, which made him hate J simultaneously because he knew J would never return his gaze. M will now suffer of only being able to say Jonathan's name per the pact, and the only way to break it is to find someone to kill him like a child or Fjord, or be forgotten entirely which could take an eternity in demon years.
Yes, I also think like you. In J's flashback, he mentioned "innocent lust + he dont care if the poison pains him". So I guess, after Wald agreed to dance with J in his bedroom, Wald (to avoid poison and pain J) decided to call out J name. So we also see a scene in which J remained alone crying in the bedroom after W and J both staying in the bedroom.
I don't get it, if love destroys a demon, how are D and S living a happy married life with a kid?