If you have braze ons don't use anything other to mount the rack. Best way is simply to use the hardware you have available and everything other will just complicate things.

I'd recommend you also turn you attention to Tubus and Rack Time rear carriers since they are at least in my opinion the best of the best. Tubus makes carriers out of chromoly, stainless steel and titanium whereas Rack Time is a sister company and makes carriers out of aluminum (like blackburn).
Both tubus and rack time have a lower attatchment system which is not adjustable but it is extremely sturdy and bomb proof, namely a thick metal plate welded directly to the rack legs.
You likely will not need adjustability if you are not using tires / fenders unsuitable for the rack in the first place. In that case you'll need to raise the rack higher than it is comfortable which may cause issues. If you want to use regular touring tires up to 45mm, use a standard touring rack. If you want to use MTB tires, use a rack that allows for wide tires. If you want to use fat bike tires, then Tubus makes racks for that as well.

The price range is around the same with blackburn and rack time, but i'd take rack time over blackburn. Although I use tubus cause steel and 45kg of carry capacity are nice features.

A medium strenght thread locking compound like blue locktite will keep screws from unscrewing themselves under vibration. You don't need much but if you want extra security, more threadlocker on the screw will make it more secure. Use the correct torque and let the compound cure for 24 hours before riding the bike to attain best results.