Standard Visitor Visa for US Citizen

November 12th 2018 my girlfriend (US Citizen) arrived in Manchester airport and was detained for 10 hours before being refused entry as they did not believe she was a genuine visitor and would not return to the US within the six months.

We contemplated applying for a Standard Visitor Visa (SVV) a month after her denial, but were told this is perhaps too soon and shows, some even said desperation, to get into the UK. We backed off a little, took our time to figure out what to do next.

December 26 2018 - January 23 2019 I visited her in the States for four weeks, and we had a great time. I plan to marry her in the USA some time next year. Since December 10 2018 she has been employed as a teacher in the USA, holding down two roles - teacher, and admin assistant.

She obviously wants to visit me in the UK for three weeks in May 2019, so she'll be applying for a SVV just before then.

I understand why she was denied, she had no return flights as we agreed to book them once she was in the UK after we decided if I was going to the USA with her, and she had a lot of stuff - a single bag of that were gifts for my family from when she and I traveled China through mid-2018.

Anyway, circumstances have changed, we're both a lot more knowledgeable on immigration law and understand why they make these decisions. To support her SVV application, she'll be including a letter of leave from her teaching position for three weeks, a letter of invitation from me, and a cover letter.

Since we plan on marrying next year in the States, we're going to get engaged when and if she comes to the UK. Is this even worth mentioning to the ECO? Or the border officer? I'm worried they may thing if we get engaged in the UK we could get married here, although I'm certain most registry offices require four weeks (28 days) notice before you can get married there which is impossible if she comes to visit for less than 28 days.

Is there anything else we should mention on her application, or advice on when to apply for a SVV? Come the end of April 2019 it will have been five months since her denial, and as I said, I visited her in the USA for a month (not sure if they'll know that, or even care).