the corners feet are fine
Sometimes i have my corners line up facing the QB with the outside foot up depending on the play. If its press then i dont tell them to do that but if i have them off around 3-5 i sometimes have them line up that way so they can see run first and they are fast enough to recover and react.
If you're a corner playing up close, your inside foot is supposed to be up, the reasoning is so that you can cover the slant easier. Lets say you're the left corner (looking at it from the defensive side), your right foot should be up so that your 1st step back is with that right foot and then 2nd with the left foot and if they are running that quick slant you pivot off that outside foot. If they run a fade or outside pattern, your shoulders are already facing that direction so its easier to turn and run with them that way. If you had your outside foot up and the WR ran a fade or outside pattern then your shoulders are facing the wrong way and you're wasting motion by having to turn your body. Now if you're playing off a little you can play on the outside shoulder with your outside foot up and your shoulders sort of facing the QB because you have a little space to recover if they run a fade or outside rout and youre in a position to read the QB and react if its a quick slant.
But when up close, inside foot (foot closest to the QB) is supposed to be up and you punch with your inside hand on the WR's inside shoulder.
If he had his outside foot up then all the WR has to do is run something quick to the outside and the DB is done. When your outside foot is up, your outside shoulder is up and therefore anything run to the outside requires you to turn your body that way which can put you off balance and is wasting motion. So all it would take is a fade or quick out and most likely the DB will be late. With his inside foot up he is protecting anything to the outside.
Now in terms of covering the slant, if its quick enough then his steps are 1. inside foot back 2. outside foot back pivot off the outside foot coming forward. If its a little deeper of a slant then its 1. inside foot back 2. outside foot back 3. inside foot back 4. outside foot back pivot off the outside foot.