KEG: Weak play throughout by Mortimer, who got the inferior game in the opening and ran his position downhill from there.
Mortimer's tentative play against Marshall's Petroff Defense quickly forfeited any opening advantage as White. Marshall's 9...Nh5 was a doubtful move (9...Nd4 was best) but was justified by Mortimer's very poor 10. Kh2.
Marshall should probably have played the more dynamic 14...d5, but his 14...Qf6 allowed him to weaken Mortimer's King-side pawns after the latter played 15. d3 (instead of 15. NxN) and 16. c3 (more tentative play!)
Nonetheless, after the exchange of Knights and Queens on moves 16 through 18, Mortimer, though having the inferior endgame, had a salvageable position. But he quickly ruined his chances.
I agree with the Tournament Book that Moritmer's 23. Re2 was not best, but the right move was 23. a4 and not the suggested 23. Rd2 which--though better than Mortimer's actual move-- would have led to problems after 23...RxR 24. BxR Rd8 25. Be3 Bb6 (rather than the Tournament Book's 25...Rd3).
But, remarkably enough, Mortimer was still in the game after the exchange of one pair of Rooks on move 25 until he played 26. Bb2 (beginning the process of burying and losing his Bishop and missing the chance to try to develop counterplay with 26. Rg1). Marshall's 26...Rd2 was not best and gave Mortimer a chance to rectify his prior move with 27. Bc1. But Mortimer completed the process of killing off his own Bishop with the dreadful 27. Ba1??
What followed was an effective moping up operation by Marshall against a floundering and clearly overmatched opponent.