Hi,
I impose change on my crypts all the time and they never melt. I can dig them up, cut their roots and re-plant them elswhere, I can put new emmersed plants in the tank and I can change the water type and they never miss a beat. If you have crypt melt after changing water then this tells you that your CO2 is not at optimum levels. If you buy new crypts and they melt after planting then this tells you that your CO2 is not at optimum levels. If you pull up a crypt and re-plant it at another location and they melt then this also tells you that your CO2 is not at optimum levels. Again, it doesn't necessarily mean that your CO2 is poor - just that it's not optimized.
The concentration of CO2 does not affect the concentration of O2. In any liquid the concentration of a gas in that liquid will depend on the pressure and concentration of that gas which is in the atmosphere above. Adding CO2 under pressure within the liquid only affects the concentration of CO2 in that liquid and has no relation to any other gas dissolved in that liquid. In other words, the behaviours of these two gases are independent.
If the concentration of a gas dissolved in the liquid is higher than atmospheric then the gas will move from the liquid to atmosphere, and vice-versa. During the photoperiod you are adding CO2 under pressure to the liquid and the plants are doing the same with O2. When the lights and CO2 go off, this stops and both gases, being at higher partial pressure in the water will find their way out of solution towards atmosphere but they will do so at their own rates.
Your sump is therefore out-gassing both of these gases until each reaches their independent equilibrium concentrations. Oxygen has a lower equilibrium concentration than CO2. So really, your sump, if exposed to atmosphere is acting as a vent. Ideally, you'd want the sump sealed against atmosphere until the concentration of O2 fell to at or below atmospheric equilibrium concentration level. Only then would it actually add O2. Since there is no inexpensive way to monitor and control this, I would seal the sump and leave well enough alone. You want to delay the out-gassing of CO2 because this makes it easier to get the CO2 levels up to optimum levels before lights on. The penalty is that O2 levels may fall below equilibrium levels - but higher CO2 levels will enable the plants to more easily replace the O2.
This is the same conundrum faced by those non-sumped tanks using an airstone at night. A bit of a Catch-22, really.
Cheers,