So to confirm, you (plural) US and EU passport holders have been allowed to board QR flights to Oman in 2019, without getting visas in advance, and were allowed to buy the visas when arriving in MCT using OMR cash?
Can you clarify the process at the new airport? Are there many people buying visas on arrival or do most people do the e-visa?
Any other airlines, and any other airports besides MCT?
Oman has largely ditched the attempts to enforce the e-visa regime for all nationalities/passport holders, and has back-tracked on the elimination of short term OMR5 visas.
Many people take up the e-visa option, i guess because it can save time on arrival. Still there's usually a queue at the VOA desk, but it's a slick operation.
I wouldn't sweat this too much. I visit frequently and I haven't experienced problems at my departure airport. No Omani immigration official has made me feel uneasy, unwelcome or quizzed me on hotel arrangements: indeed they have been consistently helpful.
I'm not sure why the UK government website suggests you should arrange a visa in advance. Both US and Canadian equivalent sites advise of the existence of visas on arrival.