I'm sure JI does drink. Whether it's as much as DB, less than, or more than, I have no idea.
As far as the "alcohol doesn't change a person" comment, I can think of 4 different ways to look at that:
1) JI was responsible for what happened to Lisa. He was drunk, lost his temper, and is now trying to convince himself, (or if caught convince others), that it was just an accident. The alcohol didn't make him into a monster that would hurt his own baby bad enough to kill her.
2) JI is completely innocent. He believes DB & believes that she wouldn't hurt, or neglect Lisa just because she was drunk. He's never hurt the kids while drunk, so why would she? He fully trusts her & believes 100% that there was a kidnapping.
3) JI is completely innocent, but is suspicious of DB. Defending DB & her drinking has become second nature,(codependents will lie just as much as addicts), but in private he's just not sure. He wants to believe her, not only because he cares about DB, but also because he doesn't want to give up hope that Lisa is still alive. He also doesn't want to admit to himself that he could be partially responsible for what happened. He trusted his baby in the hands of someone he knew had issues with alcohol.
4) JI is either fully responsible or involved in what happened, but if caught, plans on throwing DB under the bus. He is devious and manipulative, and wants people to believe number 2.