Anand usually plays the position - he is not a gambler and he is not going to take someone elses word on tactics.>

That seems unlikely to me as well, not impossible, but it's anyone's guess just what level of "respect" Anand has for Nakamura.

Not noticing 25. Rd2 ... seems like such a glaring error that seems to me only explainable as the result of some sort of <inattention>, or <flagging attention>, and this simply becomes more and more frequent as we get older ...

Anand's level of respect for Nakamura is unknown, but his age is not ...

<Many older adults notice a decline in their attentional abilities.[40] Attention is a broad construct that refers to "the cognitive ability that allows us to deal with the inherent processing limitations of the human brain by selecting information for further processing" (p. 334).[41] Since the human brain has limited resources, people use their attention to zone in on specific stimuli and block out others.>

<Classical games: Hikaru Nakamura beat Viswanathan Anand 2 to 0, with 8 draws.

Including rapid/exhibition games: Hikaru Nakamura beat Viswanathan Anand 3 to 0, with 10 draws.>

Remembering the Knight sacrifice, I was looking to reenact it here and I found at least two instances where Black can play it:

A) 23...Qd6! 24 Bc2 Nc6 25 Qg5 Qd8 26 Qh5 Nd4! 27 cd4 ed4

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Black has more than enough compensation: the weaknesses of e4 & f2; the White Bishop being shut out of the game; and the powerful pawn storm. Even my materialistic engine evaluates the position slightly in Black's favor. And all of this ignores the practical side of playing the White.

B) 24...Nc6! 25 Qg5 Nd4!? 26 cd4 ed4

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This is not as favorable to Black as the previous line (the Rook was much better posted on the f-file and the threat of Nf6 or Nh6 will force Black to waste a move on defense) but still Black has full equality and his position will be much easier to play.

The move 13 dxc5 weakens Black's d5 square. It presupposes however that White can threaten the move Nd5, and by playing his QB to e3 instead of to g5, Anand has foregone this opportunity to undermine Black's defence of this square. It seems therefore inconsistent with the move Be3.

14 Qe2 allows the attack 14...Ng4. Instead of this 14 Bg5 pins and attacks the N on f6 which defends the d5 square while 14 h3 prevents the move 14...Ng4.

After 18 Nxe3 Black has doubled e pawns but they help to defend central points and one may question whether they are not an asset to Black instead of a liability.

After 18...c4 there is a potential threat of ...Nc6 and ...Nd4! as in the game Fischer vs Kholmov, 1965 whereupon in reply to exd4, ...exd4 creates a pawn roller

The move 19 Qh5?! decentralizes the queen. In the position before 19 Qh5 the White piece which is placed the worst is the rook on a1. Therefore unless White can derive advantage from this attack, a move such as 19 Rad1 or 19 a4 is to be considered first.

Playing 17 Ne6 is also inconsistent with 13 dc5, many people think that Black's weak pawn structure is a great target for White, however the pawn on e6 is an asset for Black since it deprives White from his only viable plans in this position: Nf5 & Nd5.

It seems to me that after 12...c5

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That game was played by teletype ..

Fischer was not allowed in Havana. ..

Fischer was under extra strain than the other tournament participants.>

OK, some points:

1. I don't get how Fischer was under extra strain, he got to play from New York City instead of traveling to Cuba and after a couple of rounds he must have gotten used to the strange situation while all his opponents were playing with teletype for the first time.

2. Fischer's loss was not an accident, White's opening is theoretically bad. If you go back and look the number of people who play dc5 in the Chigorin variation falls off a cliff after 1970.

This point is perfectly valid: due to the transmissions by telex, the typical five-hour session became seven hours for Fischer. It seems reasonable to conclude that the cumulative nervous tension would almost certainly have exacted a toll in his play.

David Levy mentions this in <How Fischer Plays Chess>, stating that in his opinion, (paraphrasing) had Fischer played his games in Havana, he would have won the event by the same sort of margin as he had at Stockholm.

Levy's view in this case-unlike a number of his pronouncements I roughly recall from the book-seems reasonable to me.

As to your second point, I broadly agree: this subvariation does not look terribly impressive for White, despite the advocacy of these redoubtable players named above.