I'd look to the plugs, leads and dizzy cap since there is unburnt fuel/build up on one of the plugs. Replace the plugs regardless, once they are fouled cleaning can only do so much, also a bad plug can exadurate a bad lead.
The first test i would try will identify a miss at idle. All you have to do is to remove the lead from cyl. 1, note the change in engine noise, replace the lead and repeat on the other cyl.s. If you remove a lead and there is no change that is the plug/lead you need to replace.
If you lucky enough for it to miss when you are just revving it, lift the bonnet at night and rev away, look for spark jumping from the leads. In particular the one to the plug with the unburnt fuel/build up on it.
I'd guess the coolant usage, if not from a leak, would probably be from the head/gasket. There can be lots of different leaks on cyl. heads/gaskets.
Oil to the cyl
Water to the cyl.
Water to oil
and in some engines it's even possible to leak from the cyl. to the outside of the engine or under the valley cover on a v8.
Check your oil, is it frothey? Usually the water ends up in the oil, from my experience anyway, not with this particular engine config. If the oil is milky or frothy there is probably water in it. Another thing that can give you a clue is condensation on the filler cap when the engine is warm.
If i was you i'd get the missing problem sorted and then see how the water usage goes. It may even sort itself out once the head temp normalizes with all cyl. fireing