On the downhaul question.
The 18 ft. skiffs tie their main on the top and have realy big square top sails. It works on that boat. The new I-14 do as well. Sorry these are not catameran examples but the only differance is a rotating mast. It is the same thing you put on more downhaul, the mast bends like a bow. The sail flattens. It has more to do with the shape of the luff in the sail. A coach taught me this:
If you take a piece of paper draw a sail shape with it make sure to draw an arched luff. (Coach used a bar napkin) Then cut it out. Grab the top and bottom and pull. you should notice the flat piece of paper froms a nice bowed shaped sail. This is your mast straighting out the luff curve in your sail. Now stop pulling, this is downhaul the mast bends to follow the luff curve in the sail. and the paper goes back to being flat.
MAybe this demonstration works better at the bar.
I shouldnt matter how the sail is attached to the top of the mast it is the luff curve of the sail and the bend of the mast that creates shape in the sail.

Here is the latest redition of my Hobie18sx Main
The luff curve I'm not sure on. Just went over how important it is.HA hA
I have offsets from the original sail. Held a taught line in the center if the head and the tack. Measured the distance from the taught line to the edge if the luff rope.
Well I have the measurements not sure what to do with them yet. Working on a jib too.
Here is the main need to convert to .saildef to open with sailcut