I've never heard that too many oats would cause a problem of any kind.
As for the tough membranes, that's probably the sign of a very healthy hen. Use a knife if you have to to cut the eggs open once you crack the shell.
As for hatching eggs, you absolutely do have to make sure the humidity levels are high enough while the eggs are hatching. Problems with hatching is not caused by the membranes being too tough, as the chicks are equiped with an egg tooth that is perfectly capable of cutting through the membrane. The problems are caused by the membranes drying out and sticking to the new chick.
Also, do NOT open up the incubator to check on them. That's probably one of the biggest mistakes someone can make when hatching eggs. A chick that has just hatched has enough energy stores to live happily inside the incubator for two days while his siblings are still hatching. If you open up the incubator, you'll let all that precious heat and humidity out while your new chicks are still trying to get out of their shells/membranes.
All the books say that a chicken egg takes 21 days to hatch. However, you don't wake up on the 21st day to have had all your chicks magically pop free from their shells. It's a process.
We'll open the incubator one last time on the 18th day to check and make sure everything is ok and to turn the turners off or move the eggs to a hatching tray; depending on which incubator we're using. We'll make sure the water reservoirs are completely full. We usually won't open the incubator again until early on the 23rd day.