If I understand everything correctly (and I may not) the buffer_Write_Byte function writes to buffer 1 with index j +1 which was the last written index +1 and it puts a NULL termination at the end of it.
It does, but the value in j is the same value in i, before you reset i. Use i in the Buffer_Write_Byte() function before you reset it, and you don't need j.
Once the buffer is written function Buffer_To_Page is called and the 1 specifies that buffer 1 should be written to the page lastpage which is a variable from 0-5.
OK, so Buffer_To_Page is what commits the buffer to the flash memory. Prior to that, the data was being written to the buffer only.
So, what, exactly is this function call doing? Where, specifically, is it writing to on the page?
Never mind. You've answered that, already.
The Page_To_Buffer() method, then, is doing the reverse - getting data from the flash into a buffer, one page at a time.