how would 2p italy in felicianos react if arthur started to show signs of being suspicious of his "off" behavior
2P!Italy would be confused (and maybe frustrated), because that would mean 2P!England was going against schedule. He's the one casting the spells for the whole 2P Takeover, so he's supposed to come last. Alternatively, he thinks it's just a side effect of the worlds merging – either way, 2P!Italy would be kinda weirded out!
Arthur, on the other hand, would be in a weird position where he's harboring a parasite that's really, really bad at keeping itself hidden (and it's allies) hidden. He'd suspect "Feliciano," of course, but his mind would be in such tatters he wouldn't be able to make a convincing case – especially not against 2P!Italy, who is actually doing a fine job of imitating his counterpart. Arthur and 2P!England would also constantly be clashing for control, so his (their?) behavior would be erratic, inconsistent, and super concerning to everyone else.
I love this in-between state (the moment of possession and confronation) where you force the 1P and 2P together into one body and mind. They, as living territories, are inherently arrogant beings who treasure their autonomy VERY greatly. Who gets to keep existing? Who gets the arms, the legs, the train of thought? In many ways, it's a war between two nations (who are actually the same nation)!