Pleading the 5th, or the inability to do so, was never about state court.
Let's say Trump pardons a high ranking official from any and all matters pretaining to Russian interference in the election. Let's say his name rhymes with Mushner, just as a hypothetical.
It doesn't prevent Mushner from being charged in state court, but he could certainly plead the 5th. But that's meaningless, because it's highly unlikely that Mushner would be put on the stand by his lawyer anyways, and Schneiderman would have mountains of evidence.
So Mush is facing conviction of the same crime, or of other crimes connected to money laundering in New York.
Fucked.
Then Big ole Bob Mueller comes along and summons Mushner as a witness in the federal case another guy - let's call this one Drumpf. Mushner can't plead the 5th and has to testify. Or he could refuse and be jailed on contempt, or maybe he lies and is charged with perjury.
Conceivably Drumpf could pardon him again for those crimes, but he'd still be fucked by the state.
Or... perhaps those state charges miraculously go away, or a sentence lessened if he simply testifies.
Or maybe they don't.
Either way everyone's fucked.
You're getting a couple things crossed...its not about pleading the 5th in state or federal court...it's about the right against self-incrimination.
Where you're wrong is that Kushner could still claim the 5th, even to the special counsel, as long as what Mueller wants to him to "testify" to could incriminate him in state court. Whether Kushner would take the stand in state court is irrelevant; whether NY had a mountain of evidence already is irrelevant. You can't compel someone to testify on matters they have potential or continuing legal jeopardy on. Because anything that Kushner said to Mueller would be on the record, under oath, and could be used by NY. THAT's where the self-incrimination comes in.
So if Kusher was still in legal jeopardy in NY, he can absolutely continue to plead the 5th to Mueller.
Here's an example. Money laundering is both a Federal and State Crime.
Suppose the Feds charge you with money laundering. The President, who likes money launderers, pardons you. Feds drop all charges b/c of the pardon.
Then NY, where you did the money laundering, charges you instead.
The Feds want you to snitch on all the guys who you did your money laundering with. They still want to bust up your ring. And tell you
"since you have a pardon, you're not under legal jeopardy, so you have to spill the beans on your whole operation". Which do you (because you have a bad lawyer) and tell all to the FBI.
Then NY says
"Wow, look at this. This idiot admitted to everything, under oath, to the FBI. We've got everything we need. This guy is fucked." And proceed to use everything you said to the Feds against you in state court.
See how that works? That's why you don't lose your right of self-incrimination as long as long as you face legal jeopardy,
on a related matter, in any venue.
Now, if what NY has Kushner on has
nothing to do with what Mueller wants him to talk about, that's another story. Or if Kushner were granted immunity that anything he said to Mueller, and information potentially gleaned from it, wouldn't be used against him in NY (and this is very hard to do) that might be another matter also and he might be able to be compelled since immunity would remove the legal jeopardy in state court. This can be tricky though which is why I said earlier it was getting into the weeds. Or, as I said earlier, forget trying to
compel Kushner's testimony. Mueller simply offers him a straight up deal deal
"Give up Trump voluntarily and I'll make sure my friends in NY make the charges go away". (this IMO is the most likely scenario, but it's hard to imagine Trump's family turning against him this way, but who knows).
Or, as you alluded to, even if Kushner could theoretically be compelled to testify after a pardon (let's say there's no potential charges in NY), Trump could just pardon them again. If Kushner, after getting the pardon, was ordered to cooperate by a Federal Judge with the special counsel, and he refused, all the Judge could do is cite him with contempt and throw him in jail. And Trump would just issue a pardon for that. There's some question as to whether the pardon power extends this far (as it's being clearly used to obstruct justice), but remember it was contempt that Trump pardoned Arapaio for. So THAT would have to work it's way through he courts also.
So yes, if NY had anyone in Trump's circle for criminal activity, they could go to jail, no matter what happens with the special counsel. That's a given. And the AG and Mueller can certainly work together to apply pressure, where they're able. And everyone knows they're working together. All that's going on already with Manafort.
But a lot has to happen for any of these "NY" scenarios with Trumps kids (and Kushner) to work out in the event of a pardon...starting with them actually being investigated and charged with a crime. I'm not saying that's not happening or couldn't happen, but there's been no reports of that yet, so it's all speculation. And in any event, none of them can be compelled to testify if they have continuing legal jeopardy in NY or anyplace else on a related matter (absent a specific immunity deal).
