Yep, the tally marks were an EU invention, but one I like. That explanation for the markings on the flight helmets never sat with me, because 1) They never change, and 2) Luke gets them without anyone commenting before he's flown a single mission. Those, more than any of the other markings, I've held in abeyance for lack of clear interpretation.
As far as the whole bounty hunter thing... I have a personal notion that, as in the EU, Zuckuss and 4-LOM work together and responded to a Guild alert from the Empire, as did IG-88. Jabba would've seen the alert, too, and it would've been the first lead on Solo since Ord Mantell, so he sent Dengar and Bossk separately. Boba responded, too, but ROTJ Boba was still on Tatooine.
Yes, I'm still on that separate-people kick. Sorry. More than anything else, TBoBF reinforced for me that ROTJ Boba is an ageing clone trooper who thinks/wants to be Boba in the worst way, and -- after his falling out with real Boba -- Bossk was only too happy to set him up as a sham Boba and get him in as one of Jabba's enforcers.