Hey amolkold, I think booking UA would be more difficult for me because I have relatively few UA miles. But this is what I wound up doing.

1. EWR-HNL, nonstop, UA (40k)
2. HNL-SEA, 1 stop, AS (25k Avios)
3. YVR-JFK(stop)-LIM (30k AA)
4. LIM-JFK (????).

I need to confirm the Avios availability -- the website is glitchy and the customer service number is closed. But I was able to squeeze out the NA stopover with AA, at the sAAver level, by going from YVR rather than SEA. To me this is no big deal, because YVR is only a 3 hour drive, and my flight back is in J class on CX which sure beats domestic F.

I have some time to figure out how to get back from LIM. But here I think your suggestion might work (LIM-EWR(stop)-FCO-JFK), if I can get the miles there.