A bit confused here, why would this be set up to have both dry to the tips (water jacketed) risers and manifold dumps? Transom pickup + crossover, possibly for water pressure but I don't think the risers would be a restriction. Looking to save 2 holes in the hull

Not only that...but the preferrential flow would be to the transom outlet. The feed to the manifold is right under the transom outlet, and less restriction than riser flow.

Unless there is a restrictor in the transom outlet system..and setup as a pressure reducer as you suggested.

Hard to get in someone’s head of why….

Looks like this engine has a dominator carburetor. If so, this usually means large cid and big cam. Maybe they are attempting to reduce water thru the risers for this reason…..but not really a good way to do it. Maybe this boat previously had dry exhaust ?????and confused the person who did this ???

Lots of maybe’s.

Let’s go with what is known hopefully. Can you give us engine details ?
side note: looks like all the aluminum parts have some corrosion. This boat see salt water and/or salty air ?