Not sure if you are aware, but some of the answers to your questions can be found here:
Original D&D SetSome direct feedback:
First question: Were mixed sets common?
Fairly common with the first three printings, after that, probably not as common. When they were hand assembling the early sets they would often grab from whatever remaining stock that they had on hand. Matching printings wasn't really a concern at the time. Mixing of printings also occurred after purchase, for multiple reasons.
Does that imply late 5th, thus mid-1976?
Possibly. Sets have been discovered configured this way. It is also possible that someone replaced the booklet at a later date.
Second question: The M&M volume contains a small slip of paper with the drawing of a Hobgoblin and the wording “Hobgoblin p.35”. It was inserted between pages 34 and 35. Was that a genuine thing? Is it an errata insert? Or has someone just made their own photo-copy from a different printing and added it in?
Not aware of any official errata insert like that being released. Most likely it was added later by an owner.
Third question: Is there any difference between the boxes for the 4th and 5th printings?
Nothing significant has been noted.
Fourth question: Is there any difference in the Reference Sheets among any of the printings?
If you haven't seen this commentary yet, it pretty much describes the differences:
The Reference Sheets booklet (actually, just loose pages stapled together), was exclusively available in the Original D&D Set. Nearly every set we've heard about has a booklet with varying pages, from 6 to 12; as far as we know, each printing has the same info, only differing in format and number of pages.Some of our members that have many copies might be able to provide more info related to your questions.
