Well, coming from a purely mixed background where I learned everything at the same time (and didn't really "differentiate" between say wrestling, bjj, judo, Small Circle, Chin Na, etc... but instead pretty much just learned "grappling;" or boxing, Muay Thai, Karate, Muay Boran, etc...but instead pretty much just learned "striking"), I would say that a beginner would do well to learn all aspects of MMA right from the get go.
I reject (and have seen plenty of examples that support my position) the notion that someone cannot become highly skilled while learning all aspects of combat sports at the same time and feel that specialization in the initial stages of learning is not required to reach a high level as an MMA fighter. However, again, once a fighter builds a solid base in each area they may require (or even just prefer depending on the opponent) to focus more on certain aspects than others. Some people are "natural" strikers, and some are "natural" grapplers.
So, to answer your first question I would probably split up the hours evenly between "striking," "clinch fighting" (which would include both stand up grappling/takedown/positional work and striking skills like "dirty boxing" and Thai clinch work), and "ground fighting" (which would include positional work, submissions and sub defense, and striking/striking defense skills). This schedule may wind up reaching a "point of diminishing returns" as you said though depending on the fighter/athlete and of course their scheduled opponent (if they choose to compete).
As far as the best "street fighting" mix... First there is a difference between a "street fight" (which is essentially mutual unsanctioned "fighting") and "self defense" (which is a more morally/legally conscious endeavor and ranges from everything from avoiding bad situations altogether to situations where the goal is survival and much more commonly includes the presence of weapons, cerebral/verbal/postural self-defense skills, multiple attackers, and dire consequences). For "street fighting" a mix of boxing, stand up grappling skills (takedown and takedown defense), and quick finish or quick get up skills if it does hit the ground is a very practical skill set. For "self defense" a much more inclusive and skill set is needed.
Finally, yes, certainly cultural realities need to be taken into account. This reality is pretty clear when you look at Martial Arts from different times and cultures throughout history. For instance Jujutsu's unarmed techniques were based mostly on attacking the joints because the opponent would likely be wearing armor, thus making striking largely ineffective. On the other hand Philipino Martial Arts like Kali, Arnis, etc...are largely based around edged weapon combat because blades are so common and such a prevalent part of those cultures.