yes osctun or osccal bit, yes i know it can be done in Mikroprog but if i have used different programmer then it can be configure in code to protect it,
it can easily done in proton plus compiler
Yes, however you must read the value of osccal before writing it.
Every single chip has its value of osccal.
For example, in the pic16F675, the value of osccal is situated in the last location of memory program.
I don't use proton and I don't know how it inserts the value of osccal, but you can try to read the value of osccal before programming the chip and then insert it in your mikroC program to be rewritten in his own location of memory program.
I have never tried but, maybe it works.
For info, you can see the document "memory programming specification" for your device, from the Microchip site.