Welcome to the forum.
The specific SAAMI standard drawing for .223 Remington is here
. Case length standard is 1.730"-1.760". Note that all dimensions for the cartridge are maximums with a minus tolerance. All chamber dimensions (lower drawing) are minimums with plus tolerance. Cartridge diameters that have no specified tolerance are maximums with an assumed tolerance of -0.008".
SAAMI standard COL is 2.125" to 2.260". This is a spec used by commercial ammunition manufacturers to ensure magazine fit and feed in all SAAMI compliant guns chambered for the round. If you use a bolt gun or are single-loading, you do not have to follow it. You can customize to your own gun's limitations. The bullet curl is called a crimp cannelure and is indeed the normal case mouth location when seating that bullet. But whether or not it is the best location for accuracy in your gun is another matter. See this
Benchmark is an excellent powder for your 55 grain bullet. Varget is a little slow for it and will be less efficient.
Benchrest primers are made by the most skilled workers in the plant and are more consistent, on average, than their standard counterparts. That is not to say one particular lot of standard might not be randomly better than one particular lot of benchrest, but it might also be randomly worse. The idea is the benchrest primers, overall, have less variance than the average lot of standard primers.
As to differences, there is no standard benchrest primer type. Some are mild, like the Federal 205M and 205MAR (harder cup to resist slamfires in AR or other floating firing pin guns), and some are more like magnum primers, like the Remington 7½. I've seen up to 5% velocity difference with a fixed powder charge between different benchrest primers documented (Charles Petty's 2006 Handloader article), so you want to assume it could be worse in some instances. If the primer is the only thing you are changing, back your established powder charge down 5% when you switch primers, and work back up while watching for pressure signs
or while measuring pressure
relative your last component combination.
If you are looking for a beginning place for how to work loads up, try this system
A lot of places don't allow tracers to be fired at all because of fire hazard, so check with your local sheriff to see if it's OK with him. I doubt the type of powder will matter to igniting them, but I don't think I'd be anxious to drill holes in them. That increases ignition surface area and could conceivably increase their burn rate and heat. Not sure if there would be any consequences or not. Not knowing the tracer composition, I would treat a bore that fired tracers as if it had been fired with corrosive primers, cleaning with water-based bore solvents, like Boretech Eliminator or KG-1 and KG-2.