Dany is pretty enamoured of history, I am not disagreeing with you, but I think she would waver for a moment on the thought. I don't believe she will accept him as Aegon anyway.
History says that Targaryen rulers married their sisters, so their wives were closely related and had the same background and relationship to the Iron Throne. I don't think that there were any Targaryen rulers who married outside their own House and practiced polygamy. While I can see why Dany would entertain the notion of marrying her nephew, I can't see any historical reason for accepting Arianne.
Anyway, if Aegon gets married to Arianne, he can't simply go and take additional wives later. Their marriage would be based on a marriage treaty, and the Dornish will never sign a treaty allowing Aegon to take additional wives. Trying to marry Dany would be considered a breach of contract. The Dornish would not take it well.
She'll accept him as Aegon at first, she's nowhere near smart enough to figure out the Blackfyre angle, it'll take someone to whisper it in her ear before she starts seeing it.
She is not her brother Viserys, nor is she Aerys the Mad. If she really accepts Aegon as her nephew, she will discard whispers suggesting that he is a fake. She is intelligent enough to realize that those Blackfyre rumors are to be expected, given that he travels with the Golden Company.