Your optical tube is probably covered with a metal back plate. The 3 Phillip’s-head screws are there to hold the metal plate in place. Loosen them and remove the metal plate. You should be able to see the back of the primary mirror and 2 sets of screws around it.

There's probably nothing wrong with the eyepiece: you have probably exceeded the resolving power of your telescope. A television set looks clear 10 metres away, but up close you can see the imperfections

Not for visual purposes, as the eye cannot process the real image made by the objective. The telescope may be used without an eyepiece for camera and other instruments.

Every telescope is different, but a rough rule of thumb is 30-50X per inch diameter of the objective. A good refractor may, however, use 100X/inch on bright objects, so this is not a hard rule. You can always increase the magnification above these limits, but it is pointless if you're not seeing more. This rule breaks down for larger instruments, as the distortion of the atmosphere limits practical magnification to 300X. See Usable Magnifications.

All commercial eyepieces are made with spherical elements, as these are the only type that are easily mass produced. These naturally produce aberrations, which become much worse in highly convergent light beams. There is no way to avoid all aberrations when using spherical elements. Clever eyepiece designer can, however, minimize the objectionable ones and cause others to manifest themselves in an acceptable form.

A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.

The "Kidney Bean Effect" is not the same phenomenon as the before mentioned "black spot". In some long f.l or wide angle eyepieces, it is sometimes necessary to move the eye closer to the eyepiece in order to see the edge of the field. Sometimes, when this occurs, parts of the field between the centre and the edge are cut off, as part of the quickly converging beam misses the eye's pupil. This appears to the observer as a giant kidney bean shaped dark region that meandors around the field as head moves.

Yes. Conventional thought seems to be that all the light not reflected is transmited through to the next medium. This is critial to the performance of high-element wide angle designs with many refractive surfaces.

The ghost image, and it's evil twin, the out-of-focus ghost is caused by internal reflections inside the eyepiece. The only way to eliminate these is to eliminate air-spaces in the eyepieces, as the ghost is caused by a double bounce between two lenses in close proximity. While the ghost is an annoyance, the out of focus ghost is more of an enemy, as it reduces overall contrast of the image, which determines how much detail you'll be able to see. The treatment, if not eh cure, is di-electric multicoating of the lens-facing surfaces inside the eyepiece.