1) On U2, pins 1 and 19 both must be connected to GND (or held low) in order for the input to be placed on the output of the buffer. If either pin is high, then the entire output of U2 will be tri-stated (high impedance). Floating means leaving pins NOT or unconnected, which is not good because they can "do what they want" - stay low, drift high, or worse go somewhere in-between. Just connect both 1 and 19 to GND and you will be set.

4) The DP can be set entirely in software using the added line of code OR it can be set in hardware. If you cannot change your circuit, then use the software version. Just add the IF-THEN statement shown in 4) and you should have a DP turned on.