Catechol and HQ are structural isomers, but the change of the -OH group from ortho to para substitution changes the redox activity and other properties that are relevant to its activity in a developer.
Catechol has staining and hardening properties that may be important in a holographic developer, but the significant amount of ascorbate present would reduce that activity, so it may not be as important.
Regarding the bleach, the potassium bromide's role is to provide free bromide ion for the oxidized silver to form a soluble AgBr that can then be removed by fixation. This is known as a rehalogenating bleach. The actual oxidizer in the bleach is the benzoquinone (aka p-BQ, quinone), or the oxidized form of hydroquinone. You can prepare benzoquinone from hydroquinone by oxidizing it with hydrogen peroxide in alcohol solution and filtering out the quinone crystals. Other oxidizers might work too, as is the case for more common photographic bleaches that use potassium ferricyanide.
