Fiancee has her income tax statement (P60?) that goes April-April. However, her limited company's financial year goes from August to August.
She needs 12 months of bank statements to verify her income.
Does that mean 12 months up until the time we apply? (Say, if we apply in March, February 2016 to February 2017)?
For self-employment you need the 12 months of bank statements corresponding to a FULL TAX YEAR. So, if her company financial year is August to August, she will need to provide bank statements, along with every single required self-employment document from August 2015 to August 2016.
Her income from August 2016 to February 2017 cannot be considered as it won't cover a full financial year.
Or does she need to include 12 months plus the period of her last tax returns?
She includes only the period of the financial year (August to August).
Or does it need to be the last financial year? (And if so, which one? Her personal, the April-April? Or August-August for business?)
It depends what financial requirement category she meets.
Category F: £18,600 earned in total (before tax, but after expenses) in the last full financial year... August 2015 to August 2016.
Category G: £18,600 earned on average over the last 2 full financial years (before tax, but after expenses)... so, you would need to include all the documents from August 2014 to August 2015 AND all the documents from August 2015 to August 2016.
Also, does the "not eligible for public funds" only mean me and my children (if I had any)?
Or does it include my fiancee's children (UKC), too? I'm suddenly panicked wondering if our marriage would jeopardize her disabled child's PIP application (the DLA if you are 16 or older).
No, it's only you, because you are not a UK citizen or permanent resident and are not entitled to claim public funds until you have ILR after 5 years.
However, any children you have in future will be UK citizens, and your fiance's children are UK citizens, so they are all entitled to claim public funds.