What is the linguistic difference between "وإنا لنظنك من الكاذبين" (and indeed we think you are among the liars) and "وإنا نظنك لمن الكاذبين" (and indeed we surely think you are among the liars), and which of them is stronger in conveying meaning and an accusation of lying?

No one has mentioned a difference in meaning between the two noble verses: "And indeed, we think you are among the liars" (wa innā lanaẓunnuka mina al-kādhibīn) and "And indeed, we think you are among the liars" (wa in naẓunnuka lamina al-kādhibīn). The lām in the first verse is the lām of beginning (lām al-ibtida'), and in the second, it is the differentiating lām (al-lām al-fāriqah) between the lightened in (in al-mukhaffafah) and the negative in (in an-nāfiyah), and its origin is also the lām of beginning.