Surveyors have widely ranging expertise, so it's wise to ask about that - Do they really know your boat or at least similar boats? Ask a few questions about things you know are issues on your boat and that ought to give you a clue on their having a clue.
I'll try to look at this thread later to see if I can help but if I forget, please don't hesitate to ask - Your rig's simple and mounting spreaders is easily understood and straightforward. Here's a quick overview: Your spreaders should have mounting plates bolted or screwed to the mast with one large bolt all the way through the mast to the opposite plate. This bolt goes through a section of aluminum tube, inside the mast that prevents the pressure of the bolt and the spreader from collapsing the mast at this point - That's called the compression tube and if you need to put another in, tape it with masking tape to the end of 1" PVC and wiggle it up 'till it's near the hole, then pull in place with a screwdriver, while inserting the bolt in the other side. This one big bolt takes any force and the other screws or bolts simply keep the spreader base from rotating.
Because the spreader is compression-only, it can be kept in place securely with: proper rig tension (Use a published chart, available from Loos Company: Here's instructions: How*to*use*90*&*91*Tension*Gauges*|*Loos*&*Co.*-*Cableware*Division
), as well as a clevis pin going through the base of the spreader and the spreader base and on the outer side, securely tying it with seizing wire.
Because you've got a trailer boat, you may well have aluminum pipe for spreaders and that's ok. Thick-walled 1-1/4" aluminum is actually quite resistant to buckling.
Hope this helps!