“This is a serious constitutional violation, and not just via the rulemaking clause of Article I § 5.”
This is a serious 10th amendment violation.
————————Serious—————————
Article I, Section 5, Clause 2:
Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behaviour, and, with the Concurrence of two thirds, expel a Member.
The "privilege from arrest during their attendance... and in going to and returning from the same..." implies that voting must be made in person. Otherwise, why extend the privilege to traveling to and from the session? This also infers that a member who votes by proxy or votes remotely loses the privelege because they neither traveled to/from their respective houses, and did not attended the session in person.
Since this violates the "privilege from arrest" clause, the rule on proxy voting is unconstitutional.