The white residue sounds likely to be calcium from the hard water, E1 often suggest anti flood sensor has triggered.
Remove the plinth along the bottom and see if you can see any foam or water which has got into the sump at the bottom of the dishwasher.
No the salt can last months, it only uses a small amount of salt after each regeneration.
If you dip you finger in the compartment it will taste salty, this shows that the water has absorbed the maximum amount of salt possible.
Remove the metal plate as well and see if you can see any water in the compartment under the dishwasher.
Affraid not, make sure the appliance is switched off at the wall first, I normally use a cloth or sponge then use paper towel, followed by a hair dryer\heat gun, as it needs to be as close to bone dry as you can get it.