Why is emes a good thing to mention in the 13 middos when we’re trying to get rachamim, in light of the gemara in AZ daf 4b says emes is a reason not to daven alone for musaf on RH, seemingly it is actually worse for rachamim and brings out the din.
You are asking an interesting question. In the Sefer V’halahcta B’drachav which is a commentary on Tomar Devora ftnt. 127, he asks your question; Why is emes one of the 13 middos of mercy, if emes is din? He answers that “emes” and “din” mean that if a person didn’t do according to the letter of the law, since according to the letter of the law he did wrong, he should be punished. H-shem however works with different type of emes – an emes that is all encompassing. Meaning, that He takes the person’s whole situation into account. This is much higher form of emes, called “h’emes v’hayoshor”. Such an emes is full of understanding- truthful but also understanding. Therefore it is a midah of rachamim.