Well, thats the thing, the player decides who he sends out, only the character choosen by the player who sets bounty is able to get the reward. So, first the bounty hunter has to build up his reputation, by smaller contracts before he gets the bigger ones (unless someone wants to pay a lot money for a bounty hunter that is totally unkown, thats up to him?). I have hard time to believe that all contracts would be on a character from same faction, meaning, if one bounty hunter is to build up his rep, he has to hunt characters from diffrent factions. One time he might get this help you are talking about, but second time with contract on diffrent faction member not, and so his reputation will there stop unless he acually fullfit the contract by killing this target.
On bounty hunters profile you should be able to read his previous targets, if succeed/failed, what faction targets were in, and so on.
If there is still feeling of abuse ability then, there could be a value of carried items counted with targets death, however, all those items get destroyed if the one shooting is the assigned bounty hunter and replaced by a symbol that he then must return to the sheriff station. However this might turn problematic.
About the amount of cash for a bounty, well, i dont really think someone would want to put a lot money on someone death when death means not much in this game. So i bet this will mostly go around 500 - 1000 caps maybe. For a full time bounty hunter a lot of fun and some cash on the side. Maybe even set a "stop" at 5k or even 2k.
The multi bounty setting would not work i think, and they should be set as seperate contracts. A bounty hunter can have 3 ongoing contracts at time?