Unpowered ADC128S102QML analog input behavior

I am finishing a first circuit review of a design and have found the following condition:

In our design there are several circuit boards, three of which are always powered and 2 which can be unpowered.  One of the powered boards supplies a 2.5VDC reference to all of the boards, including the two boards in their unpowered state.  In the current revision of our design, the ADC128S102QML pin #5 (one of the analog inputs) is connected to the 2.5VDC reference through a 20k resistor with a 0.01uF decoupling cap at the ADC input.

Since the ADC128S102QML is unpowered, I am assuming the sample and hold switch is open and the only portion of the ADC input circuit connected to the 2.5VDC reference are the ESD protection diodes and 3pF input capacitance.  To simulate the effect of the diode I am simply using the 1N4148 diode spice model as the ESD protection diodes.

Here are my questions:
1. Are 1N4148 diodes good enough for the simulations I am trying to do? (If not can you recommend any good diode alternatives?)

2. My simulations show 650mV at the ADC input with 110uA passing through the diode.  I know from the data-sheet that the absolute maximum voltage at the input pin is -0.3V to VA+0.3V.  This means that when the ADC is unpowered the voltage at the input should be kept to +/-0.3V.  Are there any specified limits to the current into an unpowered chip input?

3. I have also modified the simulation to adjust the max voltage at the input pin to 450mV and the max current to about 1uA.  Is this low enough voltage and current to avoid damage to the input pin?  I have not been able to design a configuration that keeps the voltage at the input less than 0.3V.