Our cat feeder has a lid with one window that goes from compartment to compartment, so any old food won't be left out 'in the open' - ie as soon as the next feeding is due it moves to the next compartment of fresh food.
We find that Jameson never finishes the food we leave out for him (when we're away) and I always leave out lots of dry food. The dilemma we're having is, my argument is that I want to leave out more food than neccessary, incase other cats come in and eat his food. OHs argument is that more food attracts more cats to come in and eat it!
As for being lonely, I think that depends on the cat. Jameson is fine being left on his own for 2-3 days and I always tell my neighbours to fuss over him and look out for him. I never want to leave him longer than that though, he is having to go into cattery in Oct when we are going on holiday.