The HA and V5 joined in reluctantly, to avoid war with Kakin, the association were the ones who proposed making the DC taboo in the first place. The only hunters who really wanted to go beforehand were Saiyuu(assumedly because he's loyal to Beyond), Ging and those 'temp hunters'.
The succession war was the PERFECT dirt on Kakin to spark international outrage and even possibly rile up Kakin's citizens. V5 could have struck some sort of deal with a more progressive prince like Halkenburg to implode the whole venture.
And even in the scenario where they decided to go along with it anyway, the Zodiacs could have enlisted some trustworthy, capable hunters (or even themselves) for bodyguard duty to ensure better odds of a stalemate, a pro-V5 prince's victory or an escape plan for the younger princes.