Honestly, it would be at least a year before we got married. And by then, my PhD funding (£14,000 /year) would have less than 12 months left anyway. So I doubt the PhD funding is going to come into play- not that its enough?
It's not enough on it's own, but if your funding is £14,000, then your partner only needs to be earning £4,600 per year for you to meet the requirements.
Although, if I make it until 2021 on a valid work or partner visa, I think I might be able to apply for citizenship via the 10 year route- having been settled here since 2011?
Yes, you may be able to. It would be ILR that you could apply for after 10 years, not citizenship though. There's no 10-year route for citizenship, it's a 10-year route for ILR.
Is it our combined salaries to meet the financial requirement? I thought it was just the sponsors, but the documentation seems to imply its both?
It's your combined salaries if you are already living in the UK on a visa with permission to work.
It's only just the sponsor's salary if you are not yet living in the UK and don't yet have permission to work.
So, for an initial fiance or spousal visa applied for outside the UK , it's just the sponsor's salary, then for every visa after that (FLR(M) and ILR), it's your combined salaries.
As you would be applying for FLR(M) in the UK as your first visa, you can use both your salaries - which is why I suggested perhaps applying earlier and using your PhD funds, because you have the majority of the income requirement already covered.
If you were to wait until you had jobs, then you would need to have worked in those jobs for 6 months before you could apply for FLR(M), or if you hadn't been working in them for 6 months, then you would have to show that you had also earned £18,600 in the previous 12 months before applying.
Of course, if you use your PhD funding, you would still need to be able to meet the income requirement again after 2.5 years, but you would have time then to find jobs - and as you can combine income, you could actually both be working at minimum wage and you could still probably meet the requirements.
If we plan ahead, we might be able to get family to loan us funds or move funds to accounts such that we can have this as backup savings if our incomes don't cut it when the time comes. However, I can't figure out how much cash we'd have to have in an account to have this work. If we were both unemployed completely for example, what would the 'magic number' of savings be for a partner visa?
If neither of you are employed, you need £62,500 in savings and the full amount must have been in your account for a full 6 months before you apply.
Your family are not allowed to loan you funds for a visa - if they give you money towards the savings, they must write a letter for the visa application stating that the money is a gift and won't be repaid.
The first £16,000 of your savings do not count. Then on top of that, you need 2.5 times the differences between your salary and £18,600.
So, if you have no income, then that's £16,000 + (2.5 x £18,600) = £62,500 in savings
If you were to only use your PhD income, for example, you would need £16,000 + (2.5 x £4,600) = £27,500 in savings.