I dont know about now but you used to be able to sign off and then have an advance on your first months wages if you were going to be paid monthly, not sure whether you had to repay it or not, I dont think you did.
No you are not. However if you're short you can get a one off 'Back to work' payment as the Job Centre's discretion and you could get a crisis loan as well.
Depends how desperate you are really and what hours you work.
If you work less than 18 hours a week for minimum wage then you can still sign on, I know people who attend college for more hours than that and still sign on - they just don't tell them.
It'll also take about 3 weeks to get payment, and you'll only recieve around 80% of it the first time around.