I think they are the age where some people woukd and some wouldn't and so you'll have to decide. It's not like 7 year olds where it's categorically wrong or 15 year olds where it's clearly okay, but a matter of you making a call and putting in place things to make it more possible if you choose to.
For example, how about speaking to those neighbours you mention they know and saying to them, that you'd like to pop out and leave the girls but wonder if they would mind the girls knowing that in the unlikely event of a crisis they can go and knock on their door? This is such a useful thing to have in place for a couple of years and off times you need to go out. Far better to ask the neighbours and then telling the girls they have that back up is reassuring.
Yes, to ringing a couple of times, including when setting off home.
And what about just deciding you will leave 30 mins early? That will probably be fine and just make it a bit more manageable.
It's good the girls are happy to stay - them being confident is the key and it would be wrong to do it if they had doubts about it. Good as long as they aren't keen to stay because they fancy getting up to mischief.
Yes to ground rules of no cooking or answering door and must answer phone. Make sure they know what to say if someone else rings (ie Mum is in shower )
Yes to clear rules about if they have to get into bed and yes to leaving some acitivities which will fill the time like a longish film which isn't scary.
This could work fine for the right children. However only do it if you're happy - this doesn't sound like something that the earth will end over if you don't attend r only go for half of it.