The books that make up the biblical canon were sometimes written long after their dramatic date, and this is especially the case with the Pentateuch. The date of composition does not = the time of the action. There are clues like at Gen 36:31, "these are the kings who reigned in the land of Edom before any king reigned over Israel." The last clause shows that this part of Genesis had to have been written some time after king Saul. You need to jettison the primitive, simplistic thinking of JWs or other religious conservatives, likely including this chap Heisner (since he quotes the NIV, he's a conservative), if you want to understand the Bible historically.
As for Deut 32:8, the last 5 chapters of Deut are known among scholars as being textually problematic. In fact, it may well be the most textually problematic large section of the Bible. The witnesses are the Masoretic Text (MT), Septuagint (LXX), the Samaritan Pentateuch (SP), and the Dead Sea Scrolls (DSS). They do and do not match each other and are all over the place in these final chapters of Deut. Sometimes it's the SP and LXX vs. the MT. Other times it's DSS and LXX vs. the SP and MT. Other times it's MT vs. all the others. All combinations are possible.
There is a difference between the composition of the works way back when and how they have been transmitted down through time, but the two factors did interplay at times. Try to find some non-Protestant conservative sources on the composition and preservation of the OT text (Eugene Ulrich at Notre Dame is good on preservation), and forget the WTS having anything intelligent to say on the matter. They haven't got a clue here. The facts and principles are way over their heads, and their theology prevents them taking an honest look at these matters.