Yes.
Quite a lot of people post surveys here for people to complete, so I don't see why not.

Yes, but for a PhD study with such a specific participant requirement this is not a good place. It's also risky to advertise widely with incentivised participation if you need such a specific demographic, since the 'noise' of people willing to BS on Skype for 10 mins for a £15 amazon voucher can quickly outstrip the number of genuine participants.

With reference to your topic, have you tried speaking with charities and healthcare services? I'd imagine that won't be easy and require substantive paperwork, but it would seem the correct route to the kind of sample and rigor you'd need for a robust PhD study.