I don't believe that Ian Curtis would've lived much longer than in OTL.
Both depression and epilepsy are both aggrevated by stress and the worse the stress the worse the epilepsy can get. There are few things as stressful as the disintegration of a marriage and the eve of a major tour that could either make or break the band. The stress would've been bad enough with his wits about him and his health being good but with those two illnesses... let's just say that, as someone who suffers from both depression and epilepsy, I suspect he would've been a virtual basket case putting on a brave face.
His health was already failing at the time he took his own life and there is some indication that his declining health may have been one of the reasons why he killed himself. Had he not killed himself I believe he still would've died within a year or two at most so I doubt very much the events in OTL would've changed much ITTL re: Ian Curtis. What would've changed is New Order's music because it would've been quite different if Ian Curtis had died of natural causes, such as an epileptic fit, rather than suicide as dying of natural causes rarely has the same traumatic impact. .
I think that's the balancing act of this "what if". Trying to get Joy Division to create some butterflies (especially in the USA) while accounting for Ian's declining health. it's a fair call to say that he just couldn't keep on touring and touring, but a few well-located live appearances in the USA is all it could take to influence some key American alt-rock musicians in a different direction to OTL.