US fiance returning to UK following earlier 6 month stay

I am a UK citizen. My U.S. citizen fiancé travelled to the UK in:

• May 2018 (10 days)
• July 2018 (9 days)

And then returned in Feburary 2019.

• 3 February – 30 July (6 months)

She had originally planned to stay for 3-4 months, but we got engaged in April 2019 (during a long weekend in Paris, the only time spent outside the UK), so she extended her stay.

Since then I have made a few short visits to the USA. We would both like her to come to London for 10 days at Christmas, but are unsure about whether she will be allowed into the UK. The rules seem ambiguous. We know each single stay cannot exceed 6 months, but not whether that is 6 months per annum. If she visits at Christmas it will be five months since she was here.

Will she be allowed entry to the UK if she tries to visit at Christmas?

Other possibly relevant factors:
• She now lives in rented accommodation in New York
• She works as an actor (so we cannot show that she is employed). She last worked in August 2019. She has an agent in NY and has been taking industry relevant courses there.
• The short to medium term plan (now – 2 years) is that she will continue to live between NY and LA, and that we'll alternate visits to the USA/UK every 8 weeks. • Longer term (1-3 years) the plan is that we will get married and she will move to London with a spousal visa.