Check the voltage from engine block to battery negative again, but also from Bat + into the fuses. The stacked terminals at the battery can hide problems.
Check bat+ to where you got the 9V at the injectors, if you are loosing power somewhere you want to check Pos to Pos and try to find all the ?? V drops. May years ago I was working on a customers problem child Volvo, it had little hidden issues that could not be found till you looked for a drop along each wire or across each bolted terminal. I have found voltage drops in crimped connectors, not the push together part but between the wire and the crimped on terminal. These can be cleaned with an acid or using CLR cleaner.