The Second Amendment isn't even really Federal law, nor is the rest of the Constitution. It's the supreme law that sits above Federal/state law and constrains what Federal/state laws can do.
(And the Tenth Amendment protects state laws significantly from Federal laws, where applicable.)
Matt Levine has gone into great detail about the legal chicanery that brought us here, but it summarizes as "it's not gambling, it's futures trading".
This seems like nonsense to you and me (and Matt Levine), but our opinions are irrelevant here. The courts' opinions are, and so far they've been coming down on the side of the prediction markets. Mostly.
So by definition - as weird as it is - it's legal. So far.
Sure, just like storing a bunch of classified docs in a Mar-a-Lago bathroom was.
You are mixing up "currently getting away with it" for legal.
Courts move slow, especially when the party being targeted has a lot of money. Them allowing Kalshi to operate isn't an endorsement, it just means they don't see what Kalshi is doing as a significant harm. The injunction to stop them from operating in Utah is still winding it's way through appeals.
Whether or not what they are doing is legal is not determined until they either settle, Utah drops the case, or the trail and appeals process determines that they are not guilty.
Obviously not. You know how you can tell? Police will stop you from doing it.
"Kalshi and its co-founders face felony charges" would seem to describe that process, yes? The same first step a fentanyl dealer would encounter?
Swap out "selling fentanyl" for widely tolerated but often still illegal jaywalking if you like.
Police will stop you from recording them, is that illegal?
Even if you are a state legislator, and federal courts are mixed, you should write the law to raise the issue salience and get the federal law tweak.
Courts really are not the end-all-be-all when we're talking about legislators making laws, that's silly.