1) There is a general diminishing returns on any particular type of action for influence gain - this is per faction per system, rather than per player, so it doesn't matter if one person runs 50 missions or ten people run 5 each. Common modelling has four "types" of positive action - trade, bounty, exploration and mission - this may not be entirely accurate around the edges but "1 trade + 1 bounty" will generally outdo "2 bounty" all else equal. What exactly "1 trade" and "1 bounty" are I'm not entirely sure of; it's not as simple as the credit values matching up!
If you're unopposed in a system, there's therefore a (fairly low) limit on how much of each type of action it's worth doing before you essentially max out your influence gain anyway. If you're opposed ... yes, there are diminishing returns, but you still need to be further along that curve than your opponent is, so it's more about keeping your types balanced than worrying about exactly where you are on the curve itself.
For CZs specifically, it doesn't really matter it it works that way or not, because it's a simple two-way win-or-lose contest against your opponent, rather than something where your margin relative to six other factions matters. Whether 10 CZ wins counts as "10 points" or "3.77 points" you still need to win 10 if your opponent wins 9. (Different CZ intensities count differently, of course, and the bond hand-ins also have an effect, but simplifying here)
(And of course, if you don't have any opposition, 1 CZ win one day, then hand the bonds in the next day, and you're 2-0 up)
2) One victory per participant would be more consistent with other bits of the game but I don't recall seeing this one tested recently to be sure either way.
It depends of course on how good your combat pilots are, too. If you have people who can speedily complete High CZs solo, then sure, they may as well start doing that at four different ones; if you have people who struggle to complete a Medium unless in a full wing, any victory is better than nothing.