Just because he always had a melody in mind when he wrote a song doesn't mean Scott always used it. They've said in the past that they'd hand a song to Scott with a melody in mind, only to have Scott take it in another direction, and usually elevate it.
But yes, some of the time Scott stuck with what Rob came up with. The verse for Glide was definitely Rob's, as was the chorus of Interstate Love Song. IIRC, Eric wrote the melody for Trippin'. It's really impossible to know how many melodies were
Scott's, Rob's, Dean's, or Eric's without sitting down with the band and asking them track by track.
As for the latest album, I seem to recall Rob saying they refrained from writing melodies because they wanted Jeff to lend his own voice to it... But I can't recall where I heard that.
EDIT: Scratch that, looks like Dean and Rob were pretty heavy into the melodies on Butterfly:
https://www.bassplayer.com/artists/robert-deleo-re-entering-the-stone-age We put down some musical ideas we had written when Chester was still in the band. We finished the music, but we had nothing vocally, and then these songs just sat around for a while. We picked them back up, formed them melodically, and then had Jeff come into my studio to see what kind of path he’d take. Typically, when Dean and I write music, we have some sense of where we want it to go melodically. Jeff came over and put his stamp on the songs, and it just clicked on his own. Usually, when you meet someone, you don’t ask them to lay their emotions out on a table in front of you right away, but that’s exactly what we did. It’s a pretty intimate, heavy thing to do with someone. That day, he came up with some great stuff, and we got really excited about it.
So more or less, sounds like they wrote their own melodies for the stuff they'd been working on with Chester, then Jeff came in and added his own ideas.
