Go through all the training on PS and there are lots of other things that you can do other then sign up offers (I have not done any of those for long time). It's not only about exploiting free bets.
They don't necessarily, but they look for betting patterns. One of the best ways to lose an account is to do a lot of each way betting on high EV races (ie the ones that profit squad highlight as extra value) - many people have lost accounts through that alone (I certainly have!). In fact I have...
It is, as it has the most markets. Smarkets is a good alternative and is cheaper commission wise but I found it less easy to use at first. But it is good once you get used to it. Once you get used to what you are doing, matchbook does 0% commission for a while once you open an account but it...
Not much. I started with about £200, most of that went to the betting exchange to lay off the bets. You can start with less to work your way through the simpler bookie sign up offers etc. The main issue with starting small, as I found, was moving money around between bookies and the exchange.