Yes, Spreader light switch is on the FB switch panel, 2nd or 3rd from stb side.
I have trick davits on my 400, here is a copy of a PM I sent to another member about them:
Overall the trick davits have been really good, though I did have to have new center plates machined out of aluminum, as I think I may have explained in the post you read? If not, shoot me another email and I'll give you more details.
Very quick to launch and retrieve the dinghy, ours is a pretty heavy rig (11' Carib and 20 hp 4 stroke suzuki) so I added a block and tackle set up which just ties off to the base of the rail on the FB deck/roof over the cockpit. It helps a ton. We can launch the dinghy in about two minutes (including removing the tie down straps and putting them away) and recover it in about 5.
I use two long ratchet straps to secure the dinghy, one from the aft most stern eye on the dinghy to a transom cleat on the big boat and the other from the interior bow eye on the dinghy to the other stern cleat. They cross each other, sort of like dock lines on the transom. I snug the dinghy tight against the transom of the big boat.
Unless your swim platform is bigger than ours, I think the dinghy will limit your access to the swim platform when it is loaded.
One downside of carrying the dinghy on the platform is that diesel exhaust blows on it and gets it dirty.