How does battery "passthru" charge power affect lifetime capacity?
Is having 100 watts of charge power and a 50 watt load simultaneously
  similar to no load and a 50 watt charger?


Yes. The battery will get the same 50W charge power in both cases. 

However, this scenario...


  the battery will be charged full within 12
  hours of daylight using a solar panel (assume 100 watts average over
  the 12 hours). So in case 2, the battery will drain down to about 50%
  SoC overnight and then slowly go back to 100% SoC during the 12
  daylight hours.


...is unrealistic. In practice the solar system probably won't be able to produce 100W for 12 hours, so the battery will have to (at least partially) power the load for more than 12 hours.