The biggest difference with Kirker Black Diamond is it is the only base I have seen that ACTUALLY HAS ITS OWN ACTIVATOR. We have a bunch of professionals that use quality paint and then add a capful of activator in it when it is not designed to take activator. That is why Black diamond bites hard so I cannot believe you will have any problem unless it is not mixed properly. As far as how you are doctoring the paint, that is what will ruin a 1000 dollar base anyway, no no sense buying a brand name. I hope you mean candy pearls, metallics and not other bases as your blending agents, but I think Kirker is a step above most "cheap" bases.
As far as expecting 30 years out of a rust repair on old stuff, that is what is asking for alot. Either way, my belief is you put your money in the clearcoat since that is what is protecting any base.