For the OP: I've bought coach tickets on a transit bus about thirty minutes before departure so it's not a big deal for coach as that is unreserved. You might be standing for a while if the train is full. You can almost always find a seat departing SAN except in rare circumstances such as baseball games and events at Del Mar fair.
Keep in mind some days they require a reserved coach seat, so before you leave on your trip walk through a purchase of a ticket to see if it's reserved or unreserved coach. If it's reserved coach that day, book sooner rather than later as they will likely sell out.
I would plan on visiting family on the trip from your origin to Fullerton unless your plan is to travel from SAN to Fullerton the day before on your return trip. I would not cut it close on the transfer to the SWC on the return trip as I've experienced one-hour delays on the Surfliner route and things can go really south (with minimal service recovery) when something bad happens.