1. Are hosts allowed to demand entry to a property without warning at nearly midnight with no warning, simply to collect keys?
I think a distinction would be made between arriving at the property and asking to enter, and actually entering. The latter generally requires appropriate notice, whereas there's no specific policy concerning what happened to you. It's unprofessional to be sure, but not illegal.
2. Are hosts allowed to chuck you out without warning ?
Yes, unless the guest has been living in the property long enough to acquire the right of tenancy, the host reserves the right to terminate the booking at any time. In this situation, Airbnb is supposed to refund the guest for the unused nights.
3. Are hosts allowed to pretend they're a woman when they're a man ? The host page is a middle aged woman but the guy who came to the property told me it was his
It's unclear whether this was a falsified identity or simply multiple people being responsible for hosting. I don't think it would be possible to prove that someone is faking their gender identity, and it would only be relevant if the rental was a shared home with a single-gender policy, which doesn't apply here.
4. Can 5 star reviews be faked? I'm confused between the reviews and the host I met
Sure, I guess you could use other accounts to make fake bookings and review yourself. But it's also possible that your experience was an anomaly.
5. Are hosts allowed to slander with reviews which are untrue ?
It really depends on whether the word choice violates the Content Policy. If you feel a review misrepresented the stay, you can post a public response that will appear alongside it.
6. How can I raise a complaint to Airbnb when it's been closed?
You can try bringing up the matter on their social media (Twitter etc) but I'm not aware of a formal appeals process for closed complaints.
7. Is their an ombudsman?
No.