OP, you think with hindsight you made bad decisions at the time, actually it wasn't entirely unreasonable, though if your relationship was that perfect, maybe a bit drastic over a drunken kiss. You are still young and idealist perfection is what we expect, as you get older, you weigh things up more before acting, an attempted failed kiss that's never going to be repeated is not a huge deal in the grand scheme.
My advice would be that you are making a bad decision currently, you have doubt about your current situation, which is why you are asking on here.
Firstly, your ex DP is not a good man, no worthwhile man would emotionally blackmail a woman to take them back by threatening suicide - that is so off the scale wrong on so many levels that it shows he has deep issues, the kiss is nothing compared to this behaviour. If he's so desperate to be in your DC's lives, he could co-parent more hours.
Your friend seemingly had nothing to gain by telling you, but risked your friendship in the name of the truth. You say they didn't get on initially, do you know why that was? This could be why she told you, so she is not Snow White in her motives. Any friend would know the shitstorm telling you would potentially cause, and with kids involved, they would not want to drop the bomb over a drunken kiss attempt. So, can you see how you were not far wrong at the start? Both their actions were wrong, a friend who cared about you and your kids welfare would at least have agonised over telling you, and probably not said anything in the end, but monitor your DP's future behaviour. They certainly would not run to you the next day with it. Expect that if you stay with DP, your friendship will remain over.
2 options, crying after the school run and drug addiction/ anti-D's. You've taken a path of high drama, this will fog your brain.
Option 1, kick him out again and work on yourself,get counselling.Option 2, work on both yourselves together. Couples and separate counselling. You both have your individual issues if you turn to opiates, and he turns to suicide threats, so there's a lot to address here. If you have a spare room, might be better he at least stays in it if you can't face life on your own. There's a lot of work to do here, it sounds like you both tend to be emotionally dramatic. It might be fixable, whichever option you chose, you can leave the door open to each other and see how the land lies after you've done the work.