well, i thought Tony's "revival" post of two days ago was in a certain motivated by this game; or then, it was a coincidence

anyway, 8.Rc1 appears to be hard to meet

to be honest, I would have preferred to see 4.Nf3 in Caruana-Carlsen to see what Magnus would play; 4...dxe5 5.Nxe5 c6 as he played in many games in the past or 5...g6 (the Kengis) as he played against Firouza in blitz games of the past week ?

Hi

Is 13...Qb8 the best move
Both 13...Nd7 and 13...Bg4 appear to be better; I am not saying that these "improvements", if they are, make the line ok for Black

After 13...Nd7 14.Bf4 Qc7 !? 15.Nb5 Qa5+ Black appears to hold; however, both 15.d6 and 15.0-0 seem to be better

Backtracking:

i) is 12...Na6 the best move?
ii) is 11...Bc8 worse than 11.Bc5
and
iii) is 9...Na6 refuted? it appears better to me than 9...dxe5

Reply to Tony Ro's suggestion to 14.h3 (Post #18)

1.e4 Nf6  2.e5 Nd5 3.c4 Nb6  4.d4 d6 5.f4 g6  6.Nc3 Bg7  7.Be3 Be6 8. Rc1 O-O  9. b3 dxe5  10. fxe5 c5  11.d5 Bf5  12.Be2 Na6 13.Nf3 Qb8  14.h3 (TonyRo) Bxe5 15.Nxe5 Qxe5  16.Qd2 Rfe8 (Kam)  I still think that black is okay.
eg. 17.O-O Bxh3  18.Bf4 Qd4+  19.Qxd4 cxd4  20.Nb5 d3 21.Bxd3 Bd7 etc. White may have a slight advantage. White has the bishop pair, but black has an extra pawn.

Developments Against the 4 Pawns Attack, Sergeev
        7.Be3 Be6 8.Rc1 Variation.
   
The 8.Rc1 variation is personally considered as one of the more troublesome lines against the Sergeev System. Recently I have discovered a continuation, which appears to be a very combatant antidote.

1.e4 Nf6  2.e5 Nd5 3.c4 Nb6  4.d4 d6 5.f4 g6  6.Nc3 Bg7  7.Be3 Be6
8. Rc1 O-O  9. b3 dxe5  10. fxe5 c5  11.d5 Bf5  12.Be2 Na6
The key aim of this knight move is to undergo some development and vacate the b8-square for a purpose, which will be mentioned later in the game.
The knight seems to be heading in the wrong direction, but the alternative 12…. N8d7 causes congestion problems. The alternative 12 …. Bxe5 ?! was previously played and discussed in an earlier post. The immediate pawn capture allows white to launch a decisive attack involving the possibility of a stunning rook exchange sacrifice.

13.Nf3 Qb8
Time to elaborate on the previous comment! The strange deployment of the queen allows a favourable capture of the white e5-pawn. 13…. Qc7?! allows white of play the annoying 14.Nb5.

14.Qd2
     The move consolidates white’s position and interestingly it is this queen move which lead to the demise of the earlier 14…. Bxe5 continuation. If black drifts into meaningless plan mode, white will suffocate black’s position as the black knights ponder the thought “What are we doing over here?”.  The important alternative 14.Bf4 and 14.O-O will be discussed at a later date.

14….Bg4 
Moving of the same bishop three times in the opening phase does appear highly suspicious, but black’s play is sound. The imposing white centre can be broken up without dire consequences.

15.O-O Bxf3  16. Bxf3 Bxe5 
Black is able to capture the pawn and simultaneously threaten the capture of the h-pawn. White now must play a defensive move.

17.g3 Nc8!?
A crucial move with the intent of setting up on the d6-square a defensive central knight outpost and a blockade against a white d-pawn advance.

18.Rfe1 Nd6
The f7-pawn is further protected and black’s position is significantly bolstered up.

19.Bh6 Bd4+  20.Kg2 Re8  21. Nb5
White won’t be giving away a pawn without a fight and thus black may have to defend very accurately for the next twenty! moves. A possible continuation is
21…. Bg7  22.Bf4 Bf8  23.Bg4 Qd8 24.Nxd6 exd6  25.Rxe8 Qxe8 26.Re1 Qd8  27.Bg5 Qb6  28.Bf6 Nc7  29.Qf2 Ne8  30.Ba1 Qc7  32.Qf6 Qd8  33.Qc3 f5  34.Bf3 Qd7  35.a3 Re8  36.Rxe8 Rxe8  37.b4 b6  38.bxc5 bxc5 39.Kf2 Qf7 slight Ŧ Black’s chances of further activating of the minor pieces and utilising the extra pawn are very good. With the black queen on the e7-square and the king on the f7-square, black is able to play Ne8 or Nh5 followed by the liberating  Nf6 or the forced exchange off of the dark square bishops.
The most likely resulting scenario, is that white will have to grovel for the next twenty moves, but manage to scrape a draw.
40.Bb2 Qe7  41.Be2 Kf7  42.Bd3 Nh5 (or 42…. Ne8) 43.Qh8 Nf6  44.Bxf6 Qxf6  45.Qxh7+ Bg7  46.a4 Qd4+  47.Ke2 Qb2+ 48.Kf3 Qa3 Advantage Black. Etc.
Further moves are seen in the Pgn file. All constructive comments are welcome.

Thoughts on 16.Qd2 instead? Seems to me like White gets a better version of your analysis, as now Bxf8 Bf4! resources are not possible, and 16.Qd2 Re8 17.Ng5! looks terrible for Black.

FWIW, I thought that Black basically had to suffer after the surprisingly forcing 11...Bc8 12.Be2 N6d7 13.Nf3 Nxe5 14.Nxe5 Bxe5 15.Bxc5 Qc7 16.Bd4! Bxh2 17.Qd2! Bf4 18.Be3 Bg3+ 19.Kf1 Nd7 20.Ne4 Be5 21.c5! Nf6 22.Nxf6 Bxf6 and now both 23.Bf4, 23.Bd4, and even 23.Bf3 are all non-trivial to deal with.