"There's also this sheet that runs across the front beam (see below)...I'd initially thought this as some sort of mast rotating limiter (for some reason) but, although it's sitting behind the ball in the photo, I've now put it in front of the ball and am wondering if it's a way to control the jib?"
Yes, that looks like your mast rotation limiter which would be behind the mast base as pictured and attached to the mast rotation lever so that it can be adjusted to whatever mast angle you want to the boat centerline (normally about 45 degrees upwind and 90 off the wind.
You can replace the halyard ring hook by ordering a new one online and attach it to the mast where the old one had been. It is the only good way to keep the mainsail fully hoisted.
You should look online for more info about your boat to help you rig it and get other useful info. Here are some web addresses to get you started:
http://www.canadianyachting.ca/boat-reviews/79-sail/3519-freestyle-474https://www.youtube.com/watch?v=PZtokcTUuI4http://photos.cowco.net/thumbnails.php?album=64https://groups.google.com/forum/#!topic/tcdyc/TiTGK_fS4bE
