I would allow at least 2 hours (pref 3). we have used KE miles to add to our DL emp ATL/NRT legs and had enough time to complete the connection comfortably.
the airport complex is HUGE--we had the convenience of partner KE in same terminal
I don't think UA can check your bags thru
give yourself a day and stay over nearby (Radissson is way in town but free shuttle) Plenty of well-priced hotels close to NRT, too.
we've done the BKK and DPS trips this way (twice going thru and staying in Incheon AirportTown--does DL still operate ICN??)

we arrived DL counter--they gave us seat request card and had us return at set time for seat assignments, then go thru passport control then security then to the gates. don't plan on shopping either