That's a mess. They're missing proper flashing, they're ganged incorrectly, and the roof pitch is too shallow (you're right about the older units requiring a steeper slope). The flatter slope at the top allows water volume to build and the gang concentrates that volume at the lower units. I'd wager the uppers leak too, the lowers just leak worse. Those skylights had four brackets at the quarter points on the sides and could be adapted to that curved surface. The current version has an integral aluminum flange on all four sides that must sit on a flat roof deck so a direct replacement won't work. You could have a roofer 'wing it' again with curb mount units, but I would contact Velux commercial and have a proper gang designed for that application. It won't be much more expensive than messing with standard units, and far less when you consider the cost of constant bandaids.