If it is a church wedding, then a jacket and tie are pretty much mandatory. Outside on the beach, in a park, or on a ride at Disneyland then you can probably forgo the jacket and tie. I would give the ghillie/jacobite shirt a wide miss-- far too costumey to be worn as a guest at a wedding.

Thinking this through, if the wedding and reception are indoors you will be sitting in air conditioned rooms, so the jacket and tie shouldn't be that uncomfortable, especially if you forgo the waistcoat.