1) Launch Disk Utility. Delete the Windows partition so it'll become unallocated. Now you should be able to merge the entire drive.
If fails, then just reinstall, during boot, wipe the whole drive and format to GUID / Journal extended
2) What Wifi device does it have? FYI, Intel is not. supported, needs to be replaced...DW1510 for just wifi, or DW1550 for wifi/bluetooth combo