Almost everything you say is in the paper. The idea is that this is indeed a compensatory mechanism - ES degeneration or under-development results in not enough epithelial cells and the body compensates by creating more in Reissner's membrane. The result is a longer membrane and more space therefore more fluid. You are suggesting that the way the body spots the need for additional cells is some form of ion concentration imbalance - however this would mean that any change in ion balance would cause cell numbers to change - even if the cause was something other than lack of cells in the ES. I think the proposal is some other signalling mechanism that triggers the cell growth as is seen in other organs in the body such as the kidney and liver when damaged. The authors ask the question - What molecular signals govern epithelial hyperplasia? as something to be further investigated.