It took me to the United booking page at that price . . But maybe it would have fallen out after? Basic Economy.

idk how many segments you want on the outbound but the inbound routing you had is def doable/available

this prices KKPMF6BQ both ways at USD726.96 (EUR665)

There aren’t many “smaller airports that will have cheaper hotel rates” on that list, although Detroit sort of qualifies — the terminal where United operates isn’t huge, and there are fairly inexpensive airport hotels that offer shuttles. They’re not particularly nice places, mind you (except for the Westin, which isn’t cheap), but they do exist.

Your second problem is going to be that you need K inventory on a late summer Sunday in order to book the fare, and UA tends to be stingy with that.

Managed to finally get it priced out in K for €620 in Google Flights and all booked now. Oddly I could select seats on some flights even though it's basic economy.

DUB - EWR - DEN - LAS - SFO

LAX - SFO - EWR - DUB

Searched
DUB - DEN
DEN - SFO
LAX - SFO
SFO - EWR
EWR - DUB

in Google flights.

https://www.flyertalk.com/forum/34599536-post9.html.

Check your fare rules. If you start outside of USA .. sometimes there's no basic economy, but it's just non changeable (or fee to change).

Or could just be the bug mentioned