It's hard to imagine a topic that couldn't be smuggled into this convention based on that mission. There are plenty of voices who would like to diminish states rights, and as you state later in your post everything from fiscal matters to healthcare to land use to culture would be on the table.
I don't think Congress, especially a hostile one, would ever consider states rights and control thereof to be a credible "single subject".
> “It’s hard to imagine a topic that couldn’t be smuggled into this convention based on that mission. “
Can you list a few specific examples that could be smuggled?
> “I don’t think Congress, especially a hostile one, would ever consider states rights and control thereof to be a credible “single subject”.”
Can you say where in the Constitution Congress has any say on the subject of the topic, single subject or otherwise?
My understanding is Congress has no role, no discretion in determining "credibility." If 34 States apply using uniform language re: "States Rights and Control of States Rights," Congress MUST "call" a Convention of the States for Proposing Amendments.
Of course, one would expect Congress to be hostile to any proposal that would limit their own power.
...[S]ome delegates—notably George Mason of Virginia—pointed out that Congress might become abusive or exceed its powers. It might therefore refuse to adopt a necessary or desirable amendment, particularly one designed to curb its own authority. Accordingly, the Framers added [to Article V] the convention for proposing amendments as a vehicle for the states to present corrective amendments for ratification while bypassing Congress. — Natelson, p. 29. Emphasis added.Congress has no role in determining subject matter or any procedural matters; e.g., voting rules, qualifications of commissioners, size of State delegations, etc., etc. And no discretion as to its duty to "call" once 34 State Applications on the same subject matter has been reached.