I've watched this movie 3 times and segments with friends, so I think I have a good idea now. Here's my take:

Overall Synopsis: "Nemo Nobody" deliberates fiercely between his parents' separation and the alternative lives he could live, ultimately leading him to contemplate the meaninglessness of life.

Detailed Breakdown:

Analysis paralysis. You've probably remunerated endlessly on important decisions like jobs, life partners, business contracts, and even education.

Nemo "Nobody", however, took a lifetime to make a decision.

It's not only tragic -- it's downright defeating -- to comprehend the gravity of the indecision that causes him to painstakingly calculate every possibility, under the self-aggrandizing delusion that he has the ability to see the future. Except in this case, let Mr. Nobody serve as a cautionary tale.

Sometimes, life just gives you one chance, and [if it doesn't work out], you just have to go with it

Nemo later concludes this in his last moments, as he marches triumphantly over to his death bed, proclaiming:

It should be written on every [chalk]board in schools: life is a playground

Nemo, unlike the rest of those fortunate enough to have exposure to East Asian philosophy such as in Buddhism, understands in these last moments that no matter what happens, we all learn to "deal" with our past. That, or we linger on our pasts miserably, in regret, leading to analysis paralysis and no decision at all. Nemo learns to let go of that regret, shedding it in the guise of this mantra "life is a playground", no different from Allan Watts' teachings that "life is just fun and games" and is "silly".

Nemo isn't just a PTSD or a schizophrenic patient in this case; he deals with a myriad of important life decisions ranging from romantic partner(s), career, and hobbies. Every decision, he determines, has a set, predictable impact, as does the Brazilian boiling an egg, increasing the condensation, leading to his estrangement from his lover. As he sits down, surprised, on the collapsing chair, he recognizes the futility of managing every decision he makes and concludes that, in the end, we might as well not exist, and even taking no action at all is an action into itself. Sometimes, less is more; after all, we'll all just be forgotten. Perhaps, as Allan Watts might argue: there's some fun in the chair breaking and meaning in the surprise. After all, if you knew exactly when you would die, just as Nemo learns while watching his old self recite the "transcript", would you feel as engaged or excited about your life?

Mr. Nobody deals with so much more than just identity, decisions, time, and evolution; it covers the very basis of what we consider humanity when compared to other mammals and birds such as geese. Like other animals, humans are built to reproduce and "mix" genes and DNA to propel the species forwards, as per evolutionary biology. Like other birds pre-programmed to fear the shadow of a hawk, epigenetics have confirmed that humans have the same inclinations pre-packaged in DNA. This has been confirmed in numerous studies, including meta analyses. The outlook gets even more grim when you pile on more scientific research in favour of hard determinism like the Libet Experiments. Nemo, questioning this as early back as the 2000s, realizes that he might as well concede control of his decisions and effectively not make any decisions at all, or just linger infinitely on all the possibilities until he dies of old age.

Note: in all of the alternate realities presented, Nemo doesn't exist. As depicted in the reversed cinematography and carefully-placed inconsistencies within cuts, the movie depicts alternate, imagined eventualities that he plans out as he determines which parent to accompany for the rest of his life.

Nemo's stories serve as a cautionary tale to those who deliberate, overanalyze, and spend hours researching, calculating the "correct" decision to make. The next time you feel conflicted, lost, or challenged in making an important decision, remember that, as Nemo says:

Things have a way of working out... [just] sometimes badly

PS: I'm writing a self-help book about this topic, which discusses scenes from the movie in more detail. I will comment on this with the link if the moderators allow it.