Don't need to worry the digitalWrite will affect the output pin before it returns, setting a pin is one cpu instruction - the fast start up of the ADC is because the way it works is to multiplex the relevant pin to the sample/hold circuit, then sample and hold it - all within the first ADC clock or two. It takes at least 10 more clocks to do the successive approximation, one for each bit.
Thanks. My latest experiment agrees. I did another experiment without digitalWrite, and got basically the same results. That supports digitalWrite happens very quickly, and the voltage changes by the time the digitalWrite statement is complete.
Using about 4 usec to compensate for the FASTADC analogRead delay, the samples time/voltage lines up with what I get externally on the oscope. (I also had a few other instructions involved, and compensated 4 usec for each of those as well).
I didn't confirm, yet, for the standard analogRead times.