Wording of the Preliminary Emancipation Proclamation

Solution 1:

Since we know which was the intended and understood reading, this is not a historical question, my question is rather: is the first reading (the non-intended one) semantically admissible? Is the intended reading semantically admissible? Is the sentence even grammatical? You are right in saying this is not a historical question, since we know the intended meaning, which is certainly semantically admissible. The first reading is also semantically admissible, and is grammatically correct as well.

Your question appears to hinge on the use of "then". In this formulation, "then" clearly means "at the stated time", whereas you suggest it could indicate the consequence of a conditional. For the latter, "then" should be set off by commas: "...the people whereof shall, then, be in rebellion against the United States,...", but this would render the sentence ungrammatical overall.

See this link also: Comma in compound/complex sentences