The current network sucks and blocks sometimes take hours and hours due to difficulty issues with digishield. This needs to be addressed before a successful POS fork can take place. The network has to be stable and predictable. While the code might be done, the testing is possibly causing some issues with this variable in mind.
No. The Mining network will cease to be and the PoS network will take over. Fixing the digishield issue isn't needed at all.
The PoS can just be coded to pick up at X block and continue the same blockchain.