1) Literally easiest thing in history. USA, Chile, Argentina, Australia are all lined up to renegotiate, and the close neighbours in the EU won't put up with no UK goods for long.
2) Some may leave, some may stay. Depends on the incentives the UK government offers. Very few of these companies pay tax, so it's only employment being affected - the workers in question can likely move with the companies in question. Finance in pounds still needs to be in the UK so it's not like the whole sector will move (yes I know this is simplified).
3) You know what happens? The size of the pie going to nations outside the EU increases - at least until the nations IN the EU decide they're missing out on good money for no good reason. Then, fair agreements are renegotiated. It's simple.
I agree with the -8% in the first 3 years forecast, but remember: GDP is just a measure of expenditure, not welfare. The government can easily throw around subsidies and inflate that figure any time they want. It has no bearing on actual standards of living.
The big price at stake here is further integration to the EU - an undemocratic, moralised state, disconnected from its constituents, with no accountability and with some incredibly scary plans for the world. If you call a little economic hiccup a big price to pay, then you've got your priorities wrong.