"For instance, does eating leftover birthday cake restrict the flow less than eating fresh birthday cake?"
Just the opposite. Fresh, tantalizing cake can be tough to pass up. Whereas dry, leftover cake is typically unappealing. Therefore, one who eats leftover cake is likely doing so just to spite organizational rulings. One could say that such an individual is purposely being BRAZEN. Such BRAZEN conduct would restrict holy spirit flow much moreso than a one time moment of fresh, moist, chocolatey weakness.
"Or masturbating while fantasizing about one's own wife restrict the flow less than fantasizing about group sex with the COBOE and all the pioneer sisters?"
This is correct. The holy spirit fantasizes about my wife too