Generally I would not put self tappers into anything that might see tension loading, and would avoid for anything other than light side loading, they will come out it is just a matter of time.
For bolting through foam you can also use an allan key to breakup the foam around the hole and then fill with bog. Carbon plate, or penny washers work well.
A track supported at the middle and ends does work as the normal high load is when the cark is at the end, the problem is if the car is in the middle and the sheet loads up e.g. in a capsize with crew landing in jib, then it will break. Your cats cradle idea may help with that, longer term I would put a foam-carbon strip from deck to track to give more support.
With my Gnav I have 3:1 on the boom, and 4:1 going down the mast.
You need to be careful where you attach the turning blocks on the boom for the gnav, if they are on the boom then there will be a high load pulling the gooseneck apart. This can be OK but the gooseneck-boom attachment must be very strong.