What is the ruling regarding someone who said, "If I do not do this thing, I will strike this person with my eye," and then did not do that thing? Will their eye [evil eye] afflict that person, whether they intended this condition or not? And does the unwillingness to afflict the person with the evil eye, despite having a fleeting thought of it, affect the validity of repentance?

What the questioner mentioned is an effect of waswasah (whisperings/obsessive thoughts). What is required of him is to stop the blameworthy speech and to supplicate for blessing for anyone he admires, for by doing so, no harm from the evil eye will afflict him. Indeed, the hadith has been authenticated: "If any of you sees something admirable in his brother, let him supplicate for blessing for him." Al-Qurtubi mentioned that supplicating for blessing averts what is feared, and exegetes and commentators on hadith have explicitly stated that invoking blessings repels the eye of the 'ain (envier) and averts its harm.