You are on foot during your crossing but do need to get to your start point, and from your finish point to Montrose and then home.
The Challenge is about walking – not swimming! So you are allowed to take a ferry either from your start point or across (but not along) large bodies of water on your route.
Your should expect to wild camp much of the time, but you can also use hotels, hostels B&Bs and campsites where they are available. This page also includes details of some special places that open up just for Challengers.
No need to carry everything you need for a fortnight. You can resupply in Highland towns and villages or send yourself resupply parcels.