Start with the simple:
1. turning the ignition key turns a shaft that runs under the steering column. That shaft is a known failure point that can cause intermittent no start but shouldn't be related to jumping. See
viewtopic.php?f=5&t=61233&p=678574&hilit=+brother#p678574 2. that shaft turns the actual ignition switch on the left side of the steering column. See same post.
3. IF both of those work then the ECU will "allow" the vehicle to be started but only if adequate battery power is available. When I say adequate I mean within the range, a relatively narrow range, that the ECU will accept.
If 1 and 2 are ok then consider #3. Clues everything goes dim to out; it starts when jumped; battery needed charging; battery did not charge adequately when driven; you added water which indicates it's not a sealed battery which translates into somewhat less than top quality AGM mat or gel sealed battery; it's a 3 year old battery and very few batteries are good enough to warranty for more than 3 years and you can pretty much count on a 3 year warranty battery dieing at 37 months; etc. I'm assuming the alternator was in fact putting out an adequate amount of power (i.e. the reading was on a gauge with actual #s not just good/not good ranges, if the latter the gauge is junk) and that the serpentine belt idler is not bouncing around which would indicate a bad alternator clutch and by extension poor alternator output. My personal bet, tests notwithstanding, is your battery is bad or going bad (may have an intermittent internal short between cells) and simply won't put out enough juice to run stuff and crank at the same time. Of course you could have a bad battery to grd or starter to grd connection or it's possible something electrical is "on" that shouldn't be when the ignition is "off" (been there done that with a glove box light stayed on when glove box was closed). You might check for battery drain with ignition "off" but if at all possible I'd swap in a loaner battery and see if the problem goes away.
FYI IMHO the minimum acceptable CCA rating for a CRD battery is 800CCA although something in the high 700 range might do.