The best formulas might be for example 90/10 Cl/Br or 10/90 Cl/Br, but the worst might be 50/50. Now, how do we get there? We have to design the right starting conditions and then run Salt + Silver in the correct ratio and at the correct concentration to get either of the first two and avoid the last one. This requires a specific setup. If one starts from scratch, then one must do it virtually trial by error and you take what you get. I did it for a few runs and nearly went broke buying silver. Then I tried AgBr and it worked first try.
The above might answer you in part. I've said before that you can take a given published formula with all of the kinks worked out such as the Brovira formulas, and come up with a slow, low contrast result with poor keeping, but it will work and work repeatably. But, to start from scratch and get everything you need, well, that isn't easy.
Right now, the Azo type emulsion will keep about 1 - 2 years as is, and the coatings will keep over a year with no visible change at room temp. The Kodabromide type coatings have kept for a year and the emulsion for 6 months or more. The problem is that I keep running out before I can test the longevity the same way as the Azo type because it is harder to make. The AgCl/Br are even harder to make if you want them to keep as I noted above and as commented on by Glafkides. He reports that a grade 5 paper can become a grade 1 paper with high fog in just a few weeks if you don't make it right and then he leaves out the step. That step is the addition of a tailored amount of a keeping agent clearly stated in the BIOS and FIAT reports (which you appear to have). I don't have my notes here and have not looked it up for a while. The reason is that the compound crystallizes out as soon as I add it to the emulsion using the method given in those reports.
Another misprint? IDK, but I cannot stabilize a Brovira type emulsion yet. (AgCl/Br or AgCl/Br/I)