Can an opthalmologist see retinal tears or vitreous detachment with ophthalmoscope?

Yes. An ophthalmologist, especially a retina specialist, can see these things with an ophthalmoscope and treat them as appropriate.
Yes. Most of these will be detected, given appropriate symptoms such as light flashing, floaters, vision changes, etc., by the use of an indirect ophthalmoscope after eye dilation. The indirect ophthalmoscope looks like a cave diving light and gives a stereoscopic, panoramic view of the back of the eye. Tears, pathology, detachments, hemorrhage can all be seen with this technique.