Where do we start.....

Let's get a few things straight here and talk in down-to-earth terms; I'm an "enlightened being", as are several other people on this site, and I don't think that the distinction you're making is going to be helpful to you. Regardless of someone's level of attainment, we all have different semantic maps and describe the same phenomenological territory in different ways, this is a problem encountered in most situations by the majority of people. This problem, in the context of discussing enlightenment, is exacerbated because the territory is not commonly discussed and involves describing subtle fluctuations in ones own mind/body organism, something not a lot of people will generally have experience of doing.
So, let me try to address your concerns. If anything I say does not make sense to you, please question me further so that I can try to explain it in a more understandable way as I think that your misunderstandings are purely down to not having read about this stuff in a way that you can clearly understand.
When we come do diagnosis, it seems that emphasis is put on the actual momentary experience of insight or afterglow sensations (vibrations, visual distortions, etc.). Cognitive effects are left out of the pictures, for being unreliable, for what I understand.
That's not true. When it comes to diagnosis, what helps is if the person can describe in
phenomenological terms, meaning a description of their subjective experience as it happens and not their thoughts about the experience, what happens when they meditation. For example, I might say "Sat down, followed the breath, it broke down into a quick pulse in my chest, noticed warmth spreading across my solar plexus, my breath became slow and stopped for a moment before coming back up accompanied by a heaviness in my chest and mental images of dead bodies...". Cognitive processed are noted too, it's not that they're unreliable, it's just that they're no more or less important than any other sensation you'll note during a sit.
A more classical question is "Are we or could someone be born enlightened?", generally answered by "yes" (which make sense for the "already there" school).
The "ruthless non-dualist", to use Kenneth Folks phrase, would argue that everyone's already enlightened. Personally, I think it's possible for someone to have the realizations attained through insight practice by all manner of other methods, it just so happens that insight practice is incredibly efficient and effective, but I am adamant that, for the vast majority of people, some sort of meditative practice, or other mental exercise, is required.
Fingers are pointed at the conditioning we undergo while being raised, almost suggesting that things would be better if we were all feral children.
It's just plain silly to suggest that "things would be better if we were all feral children", but I know what you're trying to say. Have you ever heard of Robert Anton Wilson? He appropriated and advanced Timothy Leary's "8 Circuits of Consciousness" model and I found that this was useful when it came to understanding what sort of conditioning we're talking about here. If you google "8 circuits of consciousness model" you'll see what I mean, but bear in mind that's just another map, albeit a rather interesting one which seems to line up quite well with the models discussed on here.
Comparing the experiences of a Lama that as been raised from birth to be an enlightened being to a later-comer could shed some light on this.
As above. However, this example would involve that individual having their entire conditioning process directed towards this end.
I started to see the experience of enlightenment as a meeting with a lost relative that promise you to be by your side whenever you need him. Whenever you feel lonely or down you can call him and he will cheer you up. He keep reminding you to not worry about the future, that everything is fine the way things are. He will come to visit you on your death bed dress as a clown and exclaims "Dying is not a reason to not enjoy yourself!".
Enlightenment is a change in your relationship with the world, it's nothing to do with what you're describing, which sounds more like what a magician would call "Knowledge & Conversation of the Holy Guardian Angel", an attainment which must be released before enlightenment can happen. Once you get enlightened, you see for yourself why "dying is not a reason to not enjoy yourself". If you can experience the truth that there never was such a thing as a self which then unties the perceptual knot, not just intellectually understand it, you're enlightened.
Enlightenment put things into perspective. But what happen if we can't remember that perspective? Is happiness such a subjective state of mind that it can only be talk about in relative terms (I'm happier than I use to be)?
The perceptual change that happens on attaining enlightenment is a permanent one, it does not require any effort to maintain. The happiness which comes with that is far better than what went before and does not require any conditions to support it, but negative emotions and real-life problems can still arise and require the same skillful handling as they did before.
What everything you're saying comes down to is experience, the maps and models make much more sense when you're repeatedly experience the territory for yourself. Go get enlightened, it's worth the effort and will answer all of these questions you have.