I've seen a few instances where a search/inventory pull has seemingly caused inventory availability to change on DL flights. But there was zero evidence this was specifically due to my search and identity. Further, once the class inventory availabilty changes on flights, it doesn't matter who you are or if you are browsing from different devices or anonymously. I've never seen a case where there was different inventory availabilty on flights based on your identity as some here appear to believe. I've had an ExpertFlyer account for many years so it is not difficult to check if an airline is "lying" about actual class inventory availability (at least for revenue fares).

As far as one-ways vs roundtrip pricing differences, this should only apply to revenue fares on UA as far as I'm aware (DL, however, does generally have separate roundtrip awards on routes with separate roundtrip revenue fare filings). Long-haul international fare filings are generally asymmetric with different filings applying depending on trip origin. So it's not that unusual to see cheaper fares at times when booked as one-ways vs. roundtrip. Domestic fare filings, however, are symmetric and apply in both directions of travel. So you should only see pricing as the same or less when booking as roundtrips vs. one-ways (if there are rountrip fare filings on the route and you meet the minimum stay requirements for them).