Yes, your guesses are correct. You could re-write it: Mandates [which are] issued in which the member is bound blindly and implicitly to obey, to vote and to argue for, though contrary to the clearest conviction of his judgment and conscience, are things utterly unknown to the laws of this land, and which arise from a fundamental mistake of the whole order and tenor of our Constitution. So the whole thing to the comma before 'are things' is a noun phrase that defines the kind of mandate being referred to. The commas are to separate the points in the definition rather than making it an additional information relative clause. This is probably why they used the dash and 'these are things'. If you simplify the definition it could be something like: Mandates issued in which members are required to vote against their conscience are forbidden. The constitution forbids mandates which require members to vote against their conscience.

Wow! That is an earful. In order to understand it better, I will strip away all of the subordinate phrases so that we can see the subject, active verbs, and direct or indirect objects, if any: "Mandates - are things and arise" Really, that's all that's left when you strip away the extras. The subject is "mandates" and the two active verbs in this (compound) sentence are "are" and "arise". Your guess about "were" being a missing predicate is wrong. "Issued" is simply an adjective that describes "mandates". The sentence could have been written like this: "Issued mandates,..." and there would be no change of meaning. The first "which" refers to "mandates". The second "which" also refers to "mandates" but it should be struck out. It serves no purpose whatsoever. Is the sentence grammatical? Yes, but only understandable by a computer or by a person who works very hard to decipher it. It is bad writing. The writer should have broken his rambling thoughts down into more easily digestible pieces. A major stylistic problem is the lack of balance in the compound sentence: "They are things and arise" Since "things" is a noun, I would rather see another noun, not a verb "arise" "They are things and thoughts", for example.

The original sentence is indeed complete and grammatically correct, but it is quite complex. Let’s break it down: Subject and Predicate: The subject is “Mandates”. The predicate is a combination of the verbs “are” (implied) and “are unknown” in “these are things utterly unknown to the laws of this land...”. In this case, “Mandates issued” could be interpreted as “Mandates [that are] issued”, where the verb "are" is understood or implied. The "Which" Clauses: The sentence contains two relative clauses introduced by "which" that both modify "Mandates": “which the member is bound blindly and implicitly to obey, to vote and to argue for” Modifies “Mandates” by describing the obligation placed on the member. “which arise from a fundamental mistake of the whole order and tenor of our Constitution” This "which" also refers to “Mandates” and provides an additional description. The Pronoun "These": “These” refers back to "Mandates" and introduces the predicate “are things utterly unknown to the laws of this land.” Is it a Sentence or a Noun Phrase? Your suggestion that this might be a complex noun phrase is interesting, but because of the presence of the “are” in “these are things utterly unknown...”, it makes the sentence complete. This verb creates the main clause that turns the whole sentence into a statement rather than a long noun phrase. So, your initial guess is correct: the sentence is complete and grammatically sound. The sentence structure is complex, but it’s valid.

It can be argued that it's technically a complete "sentence", with sound grammar. I will say it is not grammatically correct when treated as a complete "statement". It is a very poorly cut fragment of a speech, allegedly delivered by a politician called Edmund Burke in 1774. Here's a bit more of the speech, cut in a way that makes it a complete statement, that's much easier to understand... I'm using my own choice punctuation for maximum clarity (considering it was a verbal speech, the decision of how to apply punctuation is open to interpretation)... "To deliver an OPINION, is the right of all men. That of constituents, is a weighty and respectable opinion, which a representative ought always to rejoice to hear, and which he ought always most seriously to consider... " "But authoritative INSTRUCTIONS... MANDATES issued... Which the member is bound blindly and implicitly to obey, to vote, and to argue for, though contrary to the clearest conviction of his judgment and conscience... These are things utterly unknown to the laws of this land, and which arise from a fundamental mistake of the whole order and tenor of our constitution." You are correct in thinking "Which" and "These" refer to "Mandates". You are also correct in thinking all the words after Mandates are talking about (modifying, or elaborating on) the Mandates. But they may simultaneously also refer to "Instructions" when you see the full speech.