In a few weeks I’ll be taking a return flight to a Schengen country where I’m living for more than the 90-day tourist period. I hold a US passport and I have I have an unusual (but valid) set of entry documents for the country. I’m 100% sure that I will be able to enter the country upon presentation of these documents but I’m concerned about convincing the airport staff of that as I’ll need to do in order to get a boarding pass. I'm especially concerned as my trip begins early morning at a regional airport where I’d expect the staff to not have much experience with this sort of thing. The Travel Ready Center tells me that I’ll need to have my documents verified. Is there a United office that I could email copies of the documents to beforehand to get preapproval?

Love your user name! My favourite animal

As to your actual question, after I upload the documents, when they have not previously been approved, it can take a couple of days which is why I complete the documents when I book the travel as someone does manually look at them after the upload.

My username tells you where I was born, as you may have guessed. Do you just upload to the standard customer service page?

Even after TRC OK, not aware of any process that relieves the airport personal from their check-in responsibilities. UA agents use Timatic and you can do trial run at https://www.united.com/en/us/timatic and see if you situation is covered. You can then guide the airport agent to the right section.

Purchasing a refundable return with 90 days, may alleviate checkin issue and entry the destination country with the special entry items.

Timatic says that I'll be ok if I tell it, truthfully, that I'm a resident of the country in question. My concern is convincing the airport staff that the documents that I have establish residency. I know that they do but they are somewhat unusual.

I'm not aware of Schengen countries requiring proof of onward travel. Can't remember if I've ever done a one-way flight but I don't think they care - TIMATIC does not show a requirement for onward travel. Possible a round-trip ticket with a longer duration could raise flags. But I think an employee seeing a flight to a Schengen country will just look at your US passport and approve the docs.

I never get pre-approval from the TRC on United traveling Australia to the US. No idea why. Delta & Qantas no problem... one of those things. Mildy irritating when I have hand luggage only.

Yes just the Travel Ready Center
Well the first time I experienced it was a SYD-SFO flight after the COVID restrictions were lifted for Aussies. Most recently it was a SYD-SFO flight as my passport was reissued while I was in Australia.

I'm not sure OP has a US passport -- which could be the source of their concern... The rest of your post is 100% spot on -- OP -- Aussie passports can also enter the EU visa free -- I've found that for foreign passport holders many times you just can't check in ahead of time -- this happens to my (Japanese) wife all the time... Try not to worry...

Check OP again — says US passport holder. So I think that part is spot on too. I am guessing the document verification message OP is getting is just either because it's the first time UA has seen that passport, or it's the occasional extra verification that strikes semi-randomly. Agree, I wouldn't worry.

Oops - my bad! Given that they are Australian by birth I made the leap lol...

Happens to the best of us — and frequently to me as well! 😜

Similar to above, I have also never been asked about onward travel or proof of residency for any one-way or return travel to Schengen area. Whether it's on UA/AC/LH or even flying one-way from UK/Ireland on FR.

So long as your documents establish you may legally enter the country, UA document check is not going to flag that you plan to stay longer than the 90-day tourist period. For all they know you could have additional flights during that 90+ days that takes you to, e.g, the UK or another non-Schengen country. If anyone is going to be skeptical it's the border patrol on arrival, who might ask . . . but they'll be better positioned to acknowledge your documents allow you to stay longer. Assuming they check at all - for the same reasons.

The biggest issue I see you potentially having is leaving after 90 days, and the exit check may ask you why you overstayed the 90-day period. I trust you will have a good answer for that.

Agree - this is the only place OP's going to get asked about length of stay.