I tend to agree with everything you mentionin your postwith the exception of the above.
I don't dispute your believe that a 6V pack saved your plane but think you were just luckyrather than the 6V providing any sort of redundancy to on board power.
Loosing a cell in a battery is not just a voltage issue, its a current draw per cell issue and an internal rsistance issue.
Any one of the cells stops working and the other cells have to compensate for pushing against the great big resistor that once was a pressure pump. This results in them draining capacity faster.
I'll bet you didn;t realise you lost a cell until you landed. There would be no way of knowing would there?
Sure with a 4.8V pack the problem is immediate. With 6V the problem occurs but you only know theres a problem once the plane stops responding. Unless you radio has bi-directional telemetry tat tells you you have had a sharp on-board voltage problem, there is no safety net.
Even with telemetry you don't know if you will have enough battery to land the plane.

Redundancy in having more cells per pack is a myth. True redundancy is in having a similar pack connected in parallel that is still capable of delivering the requiredpower. But where is the problem identification and trigger? how do you know lost a pack or a cell in a pack until you land and check each pack?

so is 4.8V suitable for sport planes? YES
is there a noticable advantage in 6V for sport planes? unlikely, some say they can feel it, others can't
is there additional safety in 6V, 5 cell packs? no, a pack is cells in series and if one drops out, voltage and capacity diminish quickly due to the higher internal resistance in the pump (pack). one or two have been lucky. safety comes from having two parallel pumps (packs)