1-A: You don't have to do any major changes. Your main .c file can be copied directly in ModusToolbox, provided you check for any API/function changes in the application.
2-A: I would strongly recommend you to download and use latest ModusToolbox and BTSDKs as we have done several bug fixes and feature additions in latest software versions. We have released MTB 2.0, 2.2 after MTB1.1, so it is always suggested to go with latest versions.
3-A: Apart from ModusToolbox IDE, you can use CLI, CyBluetool, Chipload to program the module.
4-A: During API searching for modus we directed to Wiced APIs