I cannot see that Isa 59:21 implies infallibility. It is simply discussing the requirement of God's people, the "covenant" people to preach the Word and the Gospel (1 Cor 9:16). The NT is full of examples of those who preached the (inspired) words of God being imperfect.
- Paul and Barnabas had a very "sharp" dispute that led directly to their parting, unable to work together. Acts 15:36-40
- Peter behaved badly toward Gentile believers and had to be admonished by Paul. Gal 2:11
- Timothy had to be corrected about some of his habits. 1 Tim 5:23
- Jesus had to rebuke Peter about Peter's betrayal. John 21:15-19
- Jesus had to rebuke Thomas for his doubting. John 20:29
- Jesus had to very gently correct and encourage the greatest prophet in John the Baptist when he was told "blessed is he who does not fall away on account of me" (Matt 11:3, 6).
- Many of the OT patriarch made terrible mistakes and were far from infallible as is well known.
This is part of the genius of the Gospel - God uses very human vessels (2 Tim 2:21) with all their problems (Rom 3:9-18) to accomplish great things (John 14:12). Even Paul described himself as "chief of sinners" (1 Tim 1:15).
Therefore, I see Isa 59:21 simply as a prophecy that God's Word(s) would always be preached by His "covenant" people. Essentially, Jesus repeated this in Matt 28:19, 2 Tim 4:2, 2 Cor 2:17 etc. My reading of history suggests that this has always been true, despite the limitations of our humanity.