Only thing i see that i'm confident commenting on is agreeing that on your belted set of DLs your lower back wasn't set in extension at all. You squeezed your chest up, but looks like the tension didn't make it all the way back. It didn't move during the rep, but it was flexed from the get go. But then the unbelted sets after it looked fine.
(I do some RDLs during my DL warmups just to get the feeling of an extended back. Helps sometimes)