The resistive dividers you have already provide a degree of protection. For greater protection, you could increase the values, e.g. to 15K and 33K. You could also add a diode between each CD4051 input and +5V, and between each input and ground, to prevent the inputs going much above 5V or below 0V.
If some strings appear and other don't, it sounds to me that you could have run out of RAM. Do you know how much RAM your sketch needs? Have you used the F() macro to put string literals in program memory where possible?