It's a bit of a paradox, that build, because nothing that exists today will tax it, but it will come into its own four years from now, when it can still play the latest titles on full settings. But a new machine at that time that can do the same will cost about an eighth as much as this. And he'll have paid about as much again in electricity bills in the meantime, guzzling watts to produce unneeded, invisible extra FPS. And the £7k he's invested here will have depreciated to nearly nothing (because obsolete ultra-high-end hardware is completely unsellable, because it's undesirably hot, loud and power-hungry alongside equal-performing current hardware). Whereas if he invested the same money properly and bought new PCs every couple of years he'd save thousands, because the remainder of the £7k would always be appreciating rather than depreciating. In short: he turned a big number in his bank balance into a big number in 3DMark, and it is now impossible to reverse the process. Hope it was worth it. Spoiler Spoiler: it wasn't.
4 Titans in SLI is stupid and ridiculous anyway. So putting them in a tacky-pukesicle like that is perfect quite frankly.
Series gives higher head pressure, parallel gives a higher flow rate. Neither configuration is necessarily about redundancy, its about arranging pumps to increase one value or the other. (Although obviously if you do want redundancy then parallel is the way to go)
Yes, parallel configurations are generally less useful for PC watercooling. Most watercooled systems are fairly restrictive (and getting worse as waterblock designs continue to get more restrictive), so that's where series and the stacking of pressure comes into play. Parallel is only really better when you're talking about extremely low restriction setups. And in that case, you're already moving plenty of flow with the single pump. But nonetheless that system is hideous and pointless.
My only question is: Why? For $12000 I could buy a car for $7000, fix any problems for $2000 and still get parts and a new(or rather used) NEC IPS monitor for the rest of the $3000. Hell I could just space out my graphics upgrades in the next 4 generations with $3000.