Does the emphasis on the لام (lām) in the word of majesty "Allah" within the phrase "عباد الله الصالحين" (righteous servants of Allah) and the emphasis on the نون (nūn) in "أن لا إله إلا الله" (that there is no god but Allah) invalidate the prayer, and what is the ruling on previous prayers performed in this manner?

The aforementioned لحن (linguistic error) does not alter the meaning, because the "an" in "Ashhadu an la ilaha illa Allah" (I bear witness that there is no god but Allah) originally was the shaddah-bearing "anna," but its shaddah was removed for ease of pronunciation. Similarly, the lightening (tarqiq) and the thickening (taghliz) of the "lam" (L) does not change the meaning. Your prayer is valid, whether it is for missed prayers or others. However, you must strive in the future to avoid such errors.