I cannot speak for Dektol but if it is useless after one day in a Nova slot then I am surprised it sells at all. I do have experience of Ilford MG developer and my experience is that it lasts much longer than a week
I take it that you have the tubes that cover the liquid when not using it? These tubes prevent almost all the air getting to the developer. In effect they act like the amber glass you speak of. In fact unless the amber glass is full, the tubes over the slots will protect the developer better.
I print only occasionally as well and while I have never needed to do this, you could try the following: After placing the tubes over the slots, cover them with cling film, ensuring that no air gets to the liquid.
I hope the Ansco and LPD work for you but I cannot help feel that something else is wrong if Ilford developer is no good after the times you state.
The tubes are vital. Can you confirm that you have these and use them?