1. Dear Alex, to deal with your questions in the order you have asked them, firstly, for the purposes of a visa, de facto relationships of two years entitle you to a visa on the same basis as if you were married. So if you can show you were in a de facto relationship of two years, you will get a visa for a partner which is the same as a spouse. This can be evidenced by showing you lived together, shared expenses, rented a flat together, owned a car together, etc.
2. Foreign civil partnerships are recognised in the UK AND are entitled to the same rights as if you were in a de facto relationship in the UK. So, you will be treated as being partners even though this is not available in UK law.
3. There are some exceptions to the days allowed outside the UK. One is absences owing to work abroad for a uK COMPANY. So here your partner would be allowed to count absences abroad for work as being days spent in the UK. However, the days abroad, are not quite mandatory but are followed rigorously. So, exceeding these number of days is fatal to a UK citizenship application.
4. Here is a guidance for calculating absences for Indefinite Leave to Remain and settlement. You will see the exceptions, such as holidays or work absences which don't count. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/518855/ILR-continuous-periods-v13.pdf
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