That's some ambitious joinery. I think you've come up with a reasonable solution for the interlocking rails and table legs. I'll leave to others to make suggestions for those joints.
The extensions are generally nested sliding dovetails with stops. I couldn't find an image on the web - a good furniture book should have drawings.
How to go about attaching the top rails to the underside of the table is very tricky. You could anchor on only one side of the joint in the top, but that would be flimsy and the legs would be off center when the leaf was in. You could think of using dowels protruding from the bottom of the table top to rest in sockets in the rails. Then you would need to lift the top free from the base for opening and closing. One set of sockets for closed - another for open. Not the most secure setup.
More typical extensions work with rails or aprons that are cut in line with the joint in the top. They stay fixed to each end of the table - allowing the legs to move equally from the center point as the table is opened.
So I guess I'm not helping you find your solution. I can point to the source of the problem, though. Your rails and extensions are shown separate from each other when it would be much simpler to combine them.
All the best,
**Now holding auditions for a catchy new signature**