I'm sorry, you really cannot reconcile the crotchety drunken OLDER lout that we seen in First Contact, with the mild mannered somewhat earnest guy that we meet in TOS.
That we meet
two hundred years later. And think of all the transformative experiences he went through in the interim. Meeting a starship crew from the future and learning their vision of the man he would become; experiencing first contact with the Vulcans; seeing humanity rise from the depths of war into a new era of peace and enlightenment; founding a colony on Alpha Centauri and (apparently) spending much of his life there; then going off into space to die and being captured by the Companion, rejuvenated, and stuck by himself on a tiny planetoid for 150 years. It's implausible that anyone
wouldn't be changed to a profound degree by such a long and eventful life.
Is it really any more implausible than the brash, reckless, womanizing cadet Jean-Luc Picard growing up to become the serious, disciplined, reserved starship captain we met 37 years later? Or Rom's evolution from a typical if dimwitted Ferengi to a gifted engineer and social reformer?
In fact, First Contact pretty much invalidates "Metamorphosis" since Chochrane ought to know plenty about the future. His first question to Kirk ought to have been, "do you know Picard and what happened to the Borg?" (actually this creates YET ANOTHER moment where someone could have given a heads up about the Borg to Starfleet.)
When two hundred and fifty years old you reach, remember so well you will not. Seriously, how well can you remember things you learned just 20 years ago? And there's no telling what being alone for 150 years in such a limited environment can do to a person's mind.
And ENT: "Regeneration" established that Cochrane did try to warn people about the Borg, but they wouldn't believe him, so he gave up. He probably convinced himself it had never happened.
What I like about Federation is that it better fit the facts and details that we knew about Trek's 21st century and Cochrane than First Contact did.
It fits them differently, and in a more straightforward and obvious way. But there's value in defying the obvious. As a student of history, I recognize that what we think we "know" about a past era is likely to be very different from the way it actually was, filtered through generations of interpretation and evolving cultural narrative. So a story set in a past era that confounds our expectations about that era is more credible to me than one that just confirms them.
And then there's simply the more fundamental principle that stories require tension and conflict. If Cochrane had been the saint that Picard's crew expected, if he'd been together and helpful every step of the way, there would've been no tension and it would've been boring and short.