Should "have dreamed" be translated in 2nd, or 3rd Person Plural? "... you all have dreamed" ... or, "... they have dreamed"?
The word is מַחְלְמִים, which is a masculine plural participle (which might be translated as "dream" in the present, or "dreamer," but certainly not "have dreamed" - on the exact translation see below), undeclined for person. However, the word אַתֶּם before מַחְלְמִים unambiguously makes it a second person plural address, that "you" are doing, and not the prophets mentioned previously.
Is the NKJV correct to imply an active sense of either making yourself dream, or even others causing themselves to dream?
The word מַחְלְמִים is in the hiph'il form, which usually means causing someone to do something. For the translation, some ancient versions (LXX, Peshitta) ignore the different form and translate it as "dream" (as if it said חוֹלְמִים), which might be a clue to confusion as how to translate, or possibly a different text (maybe it was erased due to being interpreted as an enclitic mem: rearranging the consonantal text gives אשר אתמ-ם חלמים).
However, according to the Masorah and the Targum, which echoes the hiph'il form of the Hebrew, it seems to be saying "which you cause to dream."
Some possible explanations:
- The explanation you propose, causing dreams through substances, is very creative, and fits the grammar, but I don't recall any mention of causing dreams through substances in the Bible
- One interpretation given is that "your dreams" are the dreams of the prophets with whom Jeremiah is disputing, and they are called "your dreams" because the people listened to them, and without their encouragement they wouldn't have continued to have those dreams (Radaq). This interpretation is seen as well in the NIV translation "Do not listen to the dreams you encourage them to have."
- I would like to propose reading מְחַלְּמִים in the pi''el form, by analogy to מְאַהֵב (e.g. in Hosea 2:9), which differs from the qal אוֹהֵב "lover" with the meaning "improper lover." Maybe מְחַלְּמִים could also mean having an improper dream
Is the NASB correct to imply that Israel was no longer to listen to anyone's dreams?
No. Jeremiah said the exile would last 70 years (29:10), and told them to build houses, marry, and pray for the welfare of their city (29:5-7). Other prophets were making the claim that the exile would be short and they would return to their land soon (28:3). This is the claim Jeremiah was disputing, which is far from saying "no longer to listen to anyone's dreams."
The dreams referred to here are those of a "חֹלֵם חֲלוֹם" ("dreamer," Deuteronomy 13:2, in apposition to "נָבִיא," prophet). He was disputing very specific prophetic dreams, just as he disputes other prophets many other times in the book. The Greek and Aramaic translations tend to translate this sort of prophet as "false prophet" (unlike the Hebrew, which never says "false prophet"), and "dreams" in this context should be understood as "false dreams" - i.e., specifically the dreams Jeremiah was disputing. He is not addressing the reliability of dreams in general.