Writing data into previously unwritten addresses in W25X10CL spi data flash
Yes.
Yes.
No.




Most of such devices can only erase their contents by at least one sector at time. This is due to their hardware design. There is no other way.

Erased bits are '1'. So you can write in any byte that is 0xFF.

Further I assume that every single '1' bit can be written as '0', but not the other way. For example it's possible to write the following values into the same byte successfully in sequence: 0xFF, 0xFE, 0x7E, 0x5E, 0x5C, 0x4C, 0x44, 0x04, 0x00. Just run an experiment to verify this assumption if you need that functionality.

If you need to erase only one byte or a few in a sector, you have to read the whole sector into your controller's RAM, erase the sector, change the erased byte in RAM to 0xFF, and write the new data back.