Among good players, should one base one's play on the assumption that the opponents will play deceptively?
First, let's address some vital points:

East has a nearly complete picture of the hand as soon as his partner plays the D6 at Trick 2:


South has exactly 3 Diamonds because Partner is echoing to show on even number.
West has 5 Spades because Declarer didn't hold  up the suit at Trick 1.
All missing high cards are in South for his 15-17 rebid of 1 NT.
East knows that West holds Club Nine because that is why the Diamond suit is being played at Trick 2 instead of the Clubs.
The only outstanding question is whether South is 35 or 44 in the round Suits (Hearts &amp; Clubs). However this is irrelevant at this time because South requires at least two Diamond tricks for his contract; unless East wins with the Q to alert Declarer to the bad Diamond lie.


Hence East's win of the Ace at Trick 2 is  a mandatory falsecard; all near-expert and better players will make the play, as it is the only way to set the contract. An expert East will still make this play, smooth as silk, with doubleton AQ of diamonds.

Let's answer these questions one at a time:


No. Count the hand and learn (at least the common) mandatory false card situations. The Diamond line illustrated here is 67% (50% for the D finesse + 68%/4 for all onside Kx Club holdings).
No. See above. There are times to adopt a mixed strategy in Bridge play, but this is not one of them. Such cases are most common when holding two equivalent cards (such as a QJ holding), and one should be prepared to decide, smoothly, which one to play in a way that your opponent cannot predict. See Restricted Choice.
No - because that requires a column of its own. CA followed by low to the CJ is 74% (68% for a 3-2 break + 30%/5 for all singleton CK in a 4-1 break). Leading the CJ for a finesse is just the same 68% as the Diamond line, but with risk of additional undertricks.


Finally, to answer your question: East's play is a mandatory false card. When playing against experts, you must expect mandatory false cards to be made. 



However, even World Champions occasionally miss a mandatory false card if pressured early enough in the hand, as this hand from the 1976 Bermuda Bowl Final demonstrates. After leading a Heart to South's low ruff. West failed to rise with KC on the low Club lead by Declarer.