We've always played with alignment being equal to your characters personality and morals. It doesn't matter which order you create them in. Alignment, as I remember it from our 2e days(and I don't remember it changing in function in 3e at all) was a role playing aid. The game said "Use your alignment as a guide on how to role play your character. If you are LG, then think about what that means when you are making decisions. How would a LG person respond to this situation?" Even if you made up your character's personality in advance, you were still assigning the alignment to him that best suited the way you were planning on roleplaying him anyway. Therefore, your alignment was still a reminder of the morals of your character.
Some people weren't good at coming up with personalities so alignment helped them flesh that out: "I don't know how my character should act...wait, I'm Chaotic and Neutral. I don't care about helping anyone since I'm not good. I also like to act randomly and without pattern. Alright, I'll randomly steal that guy's pants."
I should note that the Helm of Opposite alignment pretty much said the exact same thing in its description(at least in 2e): "Your alignment becomes the opposite of what it is currently". It's fairly obvious that it's purpose was to turn good people into evil people who would backstab the party.
I think the point is that when you're alignment suddenly becomes CE, you should use the new alignment as a role playing aid to help you decide your character's actions.
The Helm of Opposite Alignment was a terrible, idiotic thing which was great if you wanted to wreck a good party (unless they could deal with it rapidly, which was entirely down to party composition). Why would you use that comparison?
Alignment in 2E was more than "role-playing aid" in how it was presented and treated by the game. Your group may have played it just as some mild thing, and ignored alignment-related rules and so on, but they were there. I agree that it largely remained that way in 3E - a lot more than an RP aid.
The problem I'm pointing out, and I note that nothing you've said goes against this, is the mindset of "My alignment tells me what to do", rather than "My character makes decisions based on his motivations, which include but are not limited to his alignment". You don't think "How would an LG person respond to this?", you think "How would my PC, who is a human male, aged 22, who grew up in a crowded city, and values life and justice, respond to this?".
Your Chaotic Neutral pant-stealer example is a very very good example of the sort of stupidity that alignment-first thinking produced. You get someone who makes no sense, behaves in a truly arbitrary and bizarre way, and could only exist in an RPG, and whose personality, if it ever needs to be explained, is really hard to explain. CN pant-stealer is the "Fishmalk" of D&D.
(As I noted, this is pretty much what I saw with Paladins played by people who didn't normally play L or G characters - they'd usually play them as bizarre arbitrary fascists trying to avoid losing their powers, whereas people who actually gave them a personality, or who were used to playing L and/or G characters extensively with the threat of losing their powers played them as actual people, and didn't cause havoc, even if they had to say no to some plans).
EDIT - To be clear, I don't hate alignment as a whole - I did once, when I was younger and scarred by this sort of idiocy, but I can see now that alignment is a pretty useful tool, so long as it is never seen as the sole motivator (or ridiculously dominant motivator) of a PC's actions. 5E looks to be doing pretty good with this - I am just a little concerned because this is the first time I've seen "alignment first" thinking in 5E, and I hope it's just to keep the entry short.
