Changes in UV-Vis spectra in host-guest complexes
Can anybody clarify which serves as evidence for formation of a host-guest complex?


There's no single, general answer to this.

As you aptly described in the body of your post, there is a wide variety of changes to the UV-Vis absorption of a host-guest system on binding of the host and guest.  The changes to the recorded spectrum could fall into any of these categories, depending on the particular structural features of host and guest, and the exact manner that they bind to one another.

AFAIK, the only way to know how the spectrum changes when host and guest bind, is to perform careful experimentation and observe the changes that occur.