Agreed, Alexander, that the ring structures in M31 are plausibly due to mergers or pass-throughs of other smaller galaxies .
Gordon et al. (as mentioned below) hypothesized that M32 passed through the plane of M31, leading to a large-scale disturbance in the interstellar medium of M31, which could have set off a moving ring-shaped wave of star formation. However...... the orbit of M32 is not, as far as I know, well determined at the present time.
Can't understand why people still call M31 a spiral galaxy; there was never good evidence for anything apart from some minor spiral arms!
The "bar" seen in the extrapolated face-on version of M31 at 24 microns (in the mid-infrared regime) may or may not be real. It is plausible that it
is actually real, but the near edge-on orientation of M31 makes this structure very foreshortened......
The bulge component of M31 is actually somewhat square (that is, the elliptical isophotes of the bulge are somewhat "boxy"), which is often taken to be the sign of a bar that is seen in an edge-on orientation.
For instance do a google search for a paper entitled : "Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy"
[I just downloaded this and some other work about the bar of M31....I am getting data indigestion!]
Here is the boxy bulge of M31, as seen in the near infrared regime, from the 2MASS survey:
Your point about various candidate nuclei of disrupted galaxies being some of today's globular clusters, is a good one.