If Viserys was actually crowned king-in-exile by Rhaella, does Aegon (or any other hypothetical child of Rhaegar) even actually have a lawful claim through Rhaegar, as opposed to via his collateral relation to Viserys?

Keep in mind being crowned and being acclaimed are not the same thing. Viserys’ crowning is in essence a claim that Viserys is the rightful king, not the final adjudication of that claim (which could take the form of a Great Council, or a sufficient number of lords pledging fealty, or the High Septon anointing him, or a combination of all three). 

So in a scenario where Viserys is relevant in Westeros and Aegon or Jon showed up, they would have a strong legal claim that the son of the eldest son inherits before the next brother. And if they had enough political support behind them, they might actually prevail.

And that’s the key thing – ultimately, this comes down to the decisions of the political elite, as opposed to the judges. When Richard Duke of York put forward his claim that he was the rightful king of England, he was probably right according to the heralds. But the House of Lords weren’t keen on him, and raised the issue of whether they could swear allegiance to him if they’d already sworn allegiance to Henry, and so on.