Regarding citric acid, it is very polar compared to acetic acid. In a color process it is essential to get acid into the coupler solvent drops to neutralize the color developer and extract it. Citric acid, I have been told, is less efficient at this due to polarity. I have never tested it so IDK. The result would be a pinkish stain and lower image stability.
A spent blix would cause darker, contaminated colors and a gradual browning or yellowing of the print. The result is similar to using spent fix on B&W prints.
I'm using a two part blix. Part 1 contains the bleach (Sodium Ferric EDTA) and part 2 the fixer (Ammonium Thiosulfate). Is is possible to use the two parts separately as bleach and fix instead of mixing it?
Yes, you can separate the two at a considerable penalty in rate of bleaching. It will take about 2x the normal blix rate for the bleach step, and the normal blix time for the fix step. To keep contamination down, you will need a wash step in between. If you don't wash, the fix will gradually become a weak blix all by itself due to carryover.