I would like to know if the orbit behaviour of this comet is realistic and if not what could be altered to make it so.
The setting: The solar system
The system is quite similar to our own
A single G-type star
the orbits of the planets are stable
the system age is a bit less than 5 billion years
-planets from the inmost ones-
3 inner rocky planets A, B, C. C is Earth-sized. Also its orbit duration and distance from the Sun can be assumed the same as Earth: 1AU, 1 year. A and B could be any size but must be visible from C.
C has 1 main moon and 2 small moonlets.
Planet D is a large giant, very much like Jupiter with several moons. It's quite far from C and E so it does not perturb their orbits. Planets E and F are large giants, similar to Uranus (but could be altered if needed).
Other planets may be added outside the orbit of F but are not relevant to the story.
The system is visited by both periodic and non periodic comets.
The comet that this story is about should have a short period (less than 200 years). Ideally the period should be somewhere around 120 / 150 years. Long enough to keep the memory, too long for anyone to see the comet twice. In its orbit it is perturbed by E, F and especially D.
Because of these perturbations the period of the comet would change over time. Halley Comet is not regular as well but over the centuries has changed from 74 to 79 years.
Its perihelion should be inside C orbit.
it should perform a series of passages ever closer to C before hitting it (or doing a very close passage)
This 'series of passages' should be multiple, covering thousands of years.
Would the period of the comet thus be increasingly shorter?
It would be a fine addition if several periods before impacting C the comet were to partially fracture and a fragment were to hit the main moon of C. Would this be possible? Wouldn't fragments stay in the same orbit of the comet and so it would be all of it hitting / missing the moon?
I would also like it to be like the 'Daylight comet'. The little people on C should be in awe at it.
Corollary doubt: how would the closer and closer passages be realistic? Wouldn't a periodic comet have impacted C long before? (the system is almost 5 billion years old).
Additional plot idea I would like a planetary alignment (possibly of C, D, E, F) to have an effect on the orbit (for the good or worse)
I ask here in worldbuilding rather than Astronomy because in this question planet sizes, orbits, moons can be adapted to suit the scenario of a comet doing several ever closer passages before hitting (or not hitting). You can use D's influence or a very near miss with one of the moons of D to justify the orbital behaviour.