Coherent and Incoherent addition of waves

Yes, both coherent and incoherent waves can be seen by the naked eye. However, we cannot detect whether the wave is coherent or incoherent. This is because the incoherent waves have different frequencies which cannot be distinguished by the human eye. We have can only see lights of specific wavelengths as per the human limit.

Yes, whether a wave is coherent or incoherent depends on its phase difference and frequency. For converting a coherent wave into an incoherent wave, you have to damage the stable phase of the wave. This can be done by making the medium tough or mixing them with incoherent waves.

Similarly, incoherent waves can be converted into coherent waves by making their frequencies same and having all of them into the same phase.