What percentage / run time would offer the longest life for the cell with chemicals levels properly maintained for a swg?
0% (OFF) would offer the longest life; perhaps the best answer is to operate the cell at the lowest possible setting for the shortest possible time period while still achieving minimum Free Chlorine for your pool. In a trouble-free pool (no algae, < .5 chloramines) that could mean 10% for an hour or two... or 75% for 8 hours. Lots of variables affect the optimal setting/hours of operation.
The minimum run time for the pump is to turn all the water in the pool once a day, so should you adjust the output to maintain 4ppm FC?
As you probably already know, 1 turnover per day is just a rule of thumb, not a commandment. Some pools need more than one turnover, a few can get by with less than 1 turnover. Many things have an impact on whether a given level of chlorine will work for your pool: Water temperature and balance, bather load (activity), water features, environment (eg. trees, pollen, dirt), use of solar cover, pump time, etc. Reference the guidelines in Recommended Levels in Pool School and then experiment with the time setting on your SWG until it works to maintain the FC that you wish to target. In general, aim for a chlorine level at least 5% of your CYA. (If CYA=50, Free Chlorine should be 2.5 ppm or more.)
Would there be any measurable benefit to running the pump longer with a lower percentage or vice versa?
The pump almost certainly uses more electricity than the SWG; therefore, there may be a cost advantage to increasing the % and reducing pump operation time. But (just to complicate your decision a bit!) there's also a potential benefit to doing the opposite -- extended pump time and lower SWG % -- because the pool water, while exposed to the same amount of chlorine in a 24 hr period, will have the chlorine being produced when it's most needed (say, during the day in sunlight.) That's why some pool owners schedule multiple run-times, to replenish chlorine that's been consumed by UV light or otherwise spent due to oxidation of contaminants brought in by swimmers.
Sorry, it's not really that complicated...