In the optical schematic diagram, generally only the two most characteristic light directions are selected from a certain point for explanation, but it does not mean that there are only these two light directions from that point. In most cases, the light in these two directions will choose a light parallel to the optical axis and a light passing through the optical center. Not only parallel light can be imaged.
Convex lens, concave lens and concave mirror all have focal points, and the light parallel to the main optical axis will "pass through" the focal point after refraction or reflection (the concave lens extends the focal point reversely); The light passing through the optical center, no matter from which angle, will not change direction or directly reflect at the same angle.
In addition, there is one point that junior high school physics has not mentioned. At the focal point, there is a plane perpendicular to the main optical axis (focal plane): when a beam of light is neither parallel to the main optical axis nor above the optical center, a parallel line of incident light passing through the optical center can be made, and the intersection of this line and the focal plane is the point where incident light must pass.
The function of concave mirror in the above picture is to gather light to enhance brightness, but how to make the target shine brightest under limited illumination? Of course, the object is placed on its focal plane, which is where the most light is concentrated. Therefore, the beam of parallel light is only used to roughly describe the point where parallel light will meet, which means that it is on the focal plane, but it does not mean that there are only parallel lights. So, in fact, concave mirror is not difficult to adjust.
In addition, I agree with your analysis that the two parallel incident beams emitted by the light source can no longer be parallel to the eyepiece. As far as those two beams of light are concerned, if they converge on the main optical axis of the objective lens on the glass slide, they also converge on the main optical axis opposite to the objective lens; When the convergence point is not on the focal length of 1 times the main axis of the eyepiece, it is impossible to refract into parallel light.