It's a bit more complicated than that and you'd need alot more info such as comms paid etc to work it out.
have a look at the premium charge portal
https://premiumcharge.betfair.com/PremiumPortal/portal as it'll give you a better idea how close you are to PC charges even though it's due to change on 12 Oct.
Basically as soon as your commission paid is less tan 20% of you winnings you'll be liable and get a £1000 PC allowance to use before the start taking any money off you.
1) Ummm depends how much commission you've paid to win that 1000
2) The PC is calculated weekly so previous weeks have no effect other than to the rolling 60 weeks which may or may not push you into pc territory , see it's not that simple to calculate
3) No rebates for losing weeks if that's what you were looking for
4) I have no idea