if one dies there is no electrical continuity any more and the battery dies
There, you make an assumption that a cell "dying" means it goes Open Circuit.....WRONG
It can go short-circuit or high -resistance as well as O/C...an open cell would leave the whole battery "dead", 'cos the "chain " of cells is broken.
A tired, high resistance "12v" battery will still read ~14V fully charged and will STILL light a few lamps etc. for a few hours with minimal volts-drop (down to ~12.....but bang a starter across it and it WILL curl up it's toes and sink to ~6-8V let go the key and recovery will be a few seconds to full volts but NO CAPACITY for large currents (around 150amps lock-draw, i'd guess)
Over-advanced ignition "could" give similar symptoms,but i'd expect to see the prop "kick-back" as opposed to just squeezing up to TDC.
Buy a cheap multimeter (or scrounge one )..check static battery volts , check cranking volts, check and compare volts at starter (main lead and if it's a pre-engaged type, fly-lead volts as well)
Starter can have a sticking brush or a load of carbon brush-dust tracking it down......otoh, it gould be pregnant (fxxed a while since)
A bit of common-sense , logical diagnosis could save you a chunk of money.....
I gave up arguing with smartasses who told me they needed"X" without explaining what led them to that belief.... flogged them "x" then gave the bad news that "well, actually, you needed a,y and z -X was OK so now you have your old one as a spare
good for the bottom line!.