The devs have not clarified if there is an upper limit to resist, such as 85% (i.e., a 15% chance for a negative status effect to proc). As far as I know there is no potency involved in applying positive effects to your allies. Finally, it is certainly possible to have tenacity exceeding 100; for example, Ackbar leader gives +10% tenacity, so you just have to mod to ≥90% tenacity. In fact, tenacity is mostly useless in this game unless you drive it high like that since most toons requiring potency to proc will be modded decently.