It is hard to assess the correctness of the NW"T" here in the two cases you cited. Verbosity was a characteristic of Frederick Franz, the chief architect of this [per]version of Scripture. It has in fact been estimated that the Watchtower Bible is almost one third larger than other English language translations. [Compare Ro 13:1 where the NW"T" uses 30 words to say the same thing that NSAB does in 25]
In 1 Jo 5:16, the word "horaw" is used in the aorist tense, and in the subjunctive mood. The main thrust of the subjunctive is to indicate that the action of the verb is possible, depending on contextual nuances, [hence our English "might" "should" "could" etc], but in main clauses or in purpose clauses, then the action does take on a more definite nuance. Thus in the aor subj, the emphasis should be on the mood of the verb, rather than the punctiliar or single action usage.
The same construction as used at 1 Jo 15:16 occurs 4 other times, and in NONE of these has the NW"T" used the expression "catch sight of".
At Lu 2:26, the aor subj is "translated" as a perfect - "had seen"
At Lu 19:4 - it is made into an infinitive in the English "to see"
At Jo 8:56 it is made into a participle, "seeing" and
At 1 Cor 8:10 is is "translated" as it should, that is, a subjunctive - "Should see"
Compare the KIT translations at each of these verses.
I think the point that John is making at 1 Jon 5:16 is that a possibility may occur, when a disciple could, or might see his brother sinning, in which case he need do something...
As far as ""undeserved kindness" is concerned, it is not necessarily wrong, but may not capture the entire range of meaning inherent in the word. It does not necessarily point to God's "kindness" or even that it is "undeserved". It is a word of blessing based on the immutable characteristic of God's love. Hence it is better translated as "favour", and God's favour is not undeserved so much as it is unmerited, or unearned. We cannot earn the favour or blessing of God, it is a free gift. Hence if one feels the need to unpack the term "Grace" which is perfectly adequate to express the Greek, then "unmerited favour" is probably better.
I rather suspect, however, that Franz's need to use "undeserved kindness" was not so much to pay attention to "accuracy" as it was a result of his own personal prejudice. It is a word much favoured by Evangelicals, whom Franz despised, hence his "purer" alternative.