Under normal circumstances, she travels to the US on the VWP (ESTA) and never has any problems. However, we are thinking ahead to a plan that might include up to 90 days in the US, and several months in Canada. Basically, wondering whether it would be a better idea for her to apply for a visitor visa in advance, even though we would not be staying longer than the VWP allows in any case.
I would be very careful about applying for a visitor visa in advance. You should only apply for one if you absolutely do not qualify to travel on the VWP.
If you do qualify to travel under the VWP on ESTA and you don't wish to stay in the US for more than 90 days, then it's likely that the visitor visa will be refused... based on the fact that you didn't need one and could have just travelled on the VWP.
But then this creates a vicious circle, because if you have a US visa refusal this will basically make you ineligible to travel on the VWP, even though the visa was refused because you were eligible... which means you will have to apply for a visitor visa to visit the US... from then on.
Do any of these factors help or hurt us:
1. I am, as a citizen, eligible to work over there. I would not be intending to take a job there, but could do some freelance work (I'm a writer and editor). Obviously this is completely legal for me, but does it make my partner look like a risk of overstaying?
I doubt it will have anything to do with it - as long as you show that you both will leave the US together within the 90 days, I don't think it will matter.
2. She is retired (although she will probably get another job at some point), so she can't point to a job back in the UK, only to the house and also some family she has here. Are these sufficient ties to the UK?
If she can show a home in the UK and also that you live there together and will be returning to your lives in the UK, then this may be enough.
Is a visa in advance a wise idea, or are there specific forms of evidence that would be helpful at the border to prove our intent?
As I said, I wouldn't try to apply for a visitor visa unless you absolutely have no other choice.
I have a visitor visa for the US, but only because I have a DUI and so am no longer allowed to enter the US on the VWP (I travelled to the US a number of times on the VWP before the DUI with no problem at all, but since I got it and have travelled on a visitor visa, I have been under a lot more scrutiny at immigration).
Applying for a visa for the US is a long-winded, and generally not particularly fun experience (especially if you have a conviction), which I would not recommend doing unless you have to.
I have applied for 4 US visas in the past, 3 of which have been since my DUI (student visa, then 2 visitor visas) and each time, it took about 4 months from start to finish to get the visa (from booking the visa interview (which can have up to a 2-month waiting time) to actually getting the visa).
It also cost me between £200 and £400 each time ($160 for the visa, £8 to book the visa appointment (£1.20/minute to call them), £100 for travel to and from London, £200 for blood tests for two of the visas due to the conviction (had to prove I was a) not an alcoholic and b) not a danger to society), £35 for a police certificate each time). Plus, when you go for the visa interview, you can expect to be at the US Embassy for 2-4 hours just waiting to be called up to an interview window.
The US Embassy website has a VWP wizard to help you figure out if you need to apply for a visitor visa:
http://london.usembassy.gov/niv/vwp3.html