I can see how running at combustion chamber temperatures that are too low can cause problems
Actually its the combustion pressure , not temperature that can cause problems.
Rings seal because the combustion pressure gets BEHIND them , forcing them into the proper contact pressure.
This seals the bore , does not allow too much blowby and the higher pressure keeps the cylinders from being burnished , a polishing action that removes the cylinders hone marks, your oil control.
Twins are great for some , but they can not be the same size as a single for an identical boat.
If the single runs 80% to 90% rated power the twins will be under 50% all the time.
Many singles cruise under 50% of rated power , which leaves same sized twins forever loafing.
Twins used at high power do not suffer , the simplest method is smaller engines on the twin.
Base engine Selection from a taxi or tractor source , rather than an industrial diesel, is a big help to both cost and light load longevity.
But don't EVER try for full rated power for hours on end,with a lawn implement diesel ,
Just an occasional prop check , no more.