"that, with him as intercessor [or, if he were intercessor], they might succeed."
Originally, I thought like you "ablative of means". Now I see (A&G, §420.4,5) it's ablative absolute, because it replaces a conditional clause or an accompanying circumstance! Thanks, justerman. Generally, "ab eo" would be ablative of means for a person,—ablative of agent, really.
Priùs, sicut putas, modi ablativum esse id putavi, nunc absolutum esse video (secundum A&G, §419c), quia pro clausulâ conditionalis aut accidenti sequenti stat. Gratias, justerman. "Ab" cum "eo" pro homo plerumquè ablativum modi, immò agentis, significat.
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.