When coal strikes in Virginia could affect the coal supply of the entire nation, Congress, rather than just the state government, had to be able to act.
Unless that action is regulating the coal traversing state boundaries the money going the other way, or there is an agreement between the states to provide coal for sale at regular intervals, no it isn't the business of Congress. If it's just about the coal in Virginia and the workers are in Virginia...no, Congress does not need to be able to act. It's Virginia's problem, not that of the federal government.
If the framers wanted it to be a limit on the federal governments power, they wouldn't have placed it where they did.
Not necessarily, and I'd argue that interpretation is fallacious. The very structure of the federal government--to say nothing of the system of state-leaning federalism created by the document and envisioned by its creators--is meant to be self-limiting. Congress has such-and-such power, but even that is checked by two other organs of the same federal government.
Your logic could easily be expanded to say "well, if the founders wanted states to have rights, they would have left it to the states to be the sole arbiter of every federal decision, but they left it to this organ of the federal government, so they must not have," which would be erroneous. They did try that, and it didn't work. This iteration of our government was their attempt at peeling back state control juuuust enough--get just enough lift under the wings--so this thing called a "government" could fly on its own, and even then they hedged their bets with the checks and balances, ratification, amendment process, etc.
The court has ruled that you need a sort of jurisdictional element (i.e., to regulate guns near schools you need to target guns that have moved in Interstate commerce....
...[the bounds of which keep expanding over the years]
And to regulate activities that are completely within one state, there needs to be "substantial" effects on interstate commerce...
...[the bounds of which keep expanding over the years]
Let's say last year I operated a racist coupon club on Facebook, right? You weren't getting my coupons from my private group unless you could prove you were 100% Romulan; Full-bloods, no halfies like Simon Tarses. I'm a bigoted prick, but hey, it's my private coupon club, and we're just doing our small little thing, no biggie.
This year, though, SCOTUS decides that, after much review and three new derived tests, clubs on Facebook are engaging in interstate commerce. Now, what level of impact on what is defined as "interstate commerce" does my club have now as opposed to last year?
If the boundaries for X keep expanding, things far-removed from X, no matter their impact now, will eventually have a heightened impact as they move closer and closer to the boundaries of X. "Substantial" in this case might as well mean "eventually".