3. Voters really ought to be able to elect whomever they please, as many times as they please ...

7. As another mini-irony, not only does the Constitution not include any such punishment scheme for violating the Constitution ...

So no. It doesn't seem like a very good idea to me at all.

Technically, there is no punishment scheme for violating the constitution but there is one for violating the oath.

Here is the relevant part where they have to take the oath (article VI, clause 3):

The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.

And here is the relevant part where they cannot be elected into a public office if they have violated the oath (article XIV, section 3):

No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may by a vote of two-thirds of each House, remove such disability.

Now, there may not be a purely objective test of how many laws, subsequently declared as unconstitutional, one senator or representative would have to vote for to have considered as violating the oath. i.e. that number would have to be a lot higher than 3. Or, maybe there would need to be other criteria introduced besides that one number.

But whatever the criteria, to address your argument #3, voters can still elect whoever they want. But if that person has already violated the oath once, it would take 2/3 of each house to let him or her serve again. That makes sense to me, as it takes a simple majority to pass [unconstitutional] laws but 2/3 majority to change the constitution.