Absolutely DON’T chamber first, you have no control over skin thickness while shaping.
I had bought Mar Bravo pre-chambered blanks in the 70’s and cut into the chambers on nearly all of them.
Mar Bravo built a Buzzy Trent blank for Brewer, I had to route out the bottom and re-skin it as Dick had hit nearly all the chamber along one side.
I shaped for Donald Takayama and he gave me a pre-chambered blank that a customer had built, I shaped right through those unknown location pre-chambers
When Rhino was selling the pre-chambered, styro filled blanks, I repaired so many of them that were sold in the San Diego area.
Lay out the plan shape on the blank as you build it, this way you can see where the planks exit and then spot glue back from the lines 4-6". This keeps from snapping off the tips of the rail pieces as you break it apart.
As for the amount of glue, only a pea sized drop at each place, about 2 feet apart, you will be amazed how well it holds and how difficult it can be to get apart. If it prematurely comes apart, add another drop of glue, better to easily be taken apart than having to beat it to death.
Completely shape your board, shy of the sanding stage, when satisfied break it apart.
When re-assembling after chambering, put only 2 or 3 sections together with rocker straight edges to clamp to. It is very hard to keep everything aligned and doing a minimal amount of sections at a time is better than sorry later.
When I finally get to the rail section, I use strips of inner tubes to hold on the rails without scarring the wood, clamps are impossible at this point on curved, soft wood.
I used to chamber all the way through the pieces from one side, but over the years have morphed it to chamber half way from each side and stagger the chambers half way along each other.
I feel there is less of a chance of busting through if you fall on a knee or elboe and with the crown of the deck, the chamber isn’t ultra thin on one edge and thick on the other. ( leave the chamber dividing walls about an inch thick, this is where many chambered boards fail)
The rails as Bill Barnfield mentioned are very difficult to manage, I tried various router bits and shapes, but nothing was satisfying as the rail was even more difficult to hold steady.
I, during the slow times in building boards have had to go back to fine cabinet building to survive the winters.
Cabinet builders have a tool called a post router or over head router.
After using this tool to do inlay patterns on cabinet doors, I realized I could employ my 12 speed drill press to do the same.
With a section of 6" aluminum angle, clamped to the bed of the drill press the distance that you want the chamber skin to be (I generally leave the rail section a bit thicker skinned than the centers 5/8"-3/4")
I set the stops to keep the 4" router bit about 1"-1 1/2" from the surface of the bed, this way the outer edge of the rail always stays the same space away from the bit.
Draft out the chambers, go in and first plunge cut with the router bit and drill press, then come back and slide the rail along the fence and clean it up, but only go in the direction the the router bit will pull the work into the fence, otherwise it will walk away from the fence and cut out throught the side of the rail.
When finished with this phase. move the fence and plunge out the ends of the chambers. I have a bitchin little tool called a scorp, it is a loop knife on the end of an exacto handle, you can go into the bottom of the rail chambers and clean them up so finely with a nice rounded bottom curve.
It taks me an entire day to chamber a long board/gun, if you are in a hurry, don’t attempt.
What do they say? “No one has time to do it right, but time to do it over”
