The biggest fireballs come from powder that is burnt after the bullet leaves the muzzle.
True.
The still-burning powder and gas is expelled out the muzzle.
True.
To avoid having most of the powder combust while the bullet is still within the barrel, a slow-burning powder that is heavily covered with deterrent is used
This is nonsense. As written it makes no sense, to me, at any rate.
I think I might know what you are trying to say, but as written I can't be sure and its incorrect as well. Nobody is "trying to AVOID having most of the powder combust while the bullet is still in the barrel" Rather we usually try for the opposite, having most of the powder burn in the barrel.
Powders like 296 are heavily coated with deterrent chemicals to retard their burn rate. NOT so they produce a large flash, but so that the slower burn gives more "push" over the time the bullet is in the barrel.
Look at the load data with short barrels and slow powders. Even though a lot of the powder seems to burn outside the barrel, the part that does burn in the short barrels generally yields higher if not the highest velocities than "faster powders". The downside is there is more than enough unburnt/still burning powder ejected from shorter barrels to give a large flash. Its not the intent, its the unavoidable result with the chemicals used and the length of barrel they burn in. There's no free lunch.
The deterrent coating used in 296 is part of what makes it burn the way it does. That burn rate gives you one size flash in a 4", a different one in a 6", and a much different one in an 18" barrel. The coating used on some powders to produce low flash loads for use in short barrel guns is a different substance and a different subject. I guessing you are confusing the two.
Back to the Coonan, my experience is that you may find you need to run light bullet loads at or near top end levels, in order to provide the recoil needed for consistent operation of the action. The industry standard weight bullet was, and still is the 158gr. Tolerances vary, every gun is an individual, but you may find you need to run light bullets (125gr etc) hot enough to produce the same level of recoil the standard 158s do. (unless you change something in the gun springs, etc.)
Personally, I wouldn't bother with W296 in the .357 Mag Coonan. I would use 2400. Its less sensitive to load density than 296, will give nearly the same velocities and will give you that big fireball you want, from a 6" or shorter barrel.