If you didn't break anything, were able to set up, launch, sail and get the boat back on the trailer without anyone getting hurt you really had a good day.
Do you launch the boat at the Cooper River Park?
The time it takes you to launch and 'de-rig' the boat will get to be less and less the more you do it.
I found this picture of an O'Day Javelin on the web: O'Day Javelin, 1973, Jacksonville, Florida sailboat for sale, yacht for sale
It looks like your main sheet has a fairly standard 'double fiddle block with becket and cam cleat' for the lower sheet hardware. The cam cleat part must be what is not grabbing the sheet (two little spring loaded jaws). Before you do anything you should see if you can clean and oil the cam cleat jaws. There may be screws that hold them in place that you can loosen and then clean out any crud that has accumulated there - just be careful you don't loose the spring mechanism that tensions them. Lots of WD 40, a Q tip or 2 and a rag may be all that it takes to get the jaws to 'bite' again so the sheet holds tight when cleated. If this piece of hardware is working it will be much easier to control your main sail and boat.
If you want to price a new unit for this look here: Garhauer Marine Hardware -1530772
but all that is wrong with your current hardware is probably a little cleaning and lubing.
As to your centerboard being a hunk of rust: I'd tackle that this winter when you don't want to be sailing. When that time comes you will want to get a wire brush attachment for a drill or grinder and grind off as much rust as you can, then treat the metal with Ospho, Naval Jelly to deactivate the rusting process, then you could either just paint it our coat it in epoxy/cloth (and paint the epoxied surface). This would also give you time to find a cheaper alternative to buying a new replacement (finding a good metal shop that will do it for a decent price is a good idea).
An empty bleach or laundry detergent bottle with the bottom cut off makes a wonderful bailer.
Congrats on your new boat!