First things first, but in this case, second things first.
- When an asteroid or moon, passes the Roche Limit, it breaks apart and forms rings around the primary body.
This is a widely believed misconception of the Roche limit. The Roche limit pertains to objects that are held together by gravitation only. Once chemical bonds come into play the Roche limit is not a limit. A solid or liquid object held together by chemical bonds as well as by self-gravitation can hold itself together well inside the Roche limit.
If there is any validity to the giant impact hypothesis, the object (or multiple objects in the case of variants of the giant impact hypothesis that posit more than one giant impact), such objects would have impacted the Earth nearly intact.
These two things got me thinking,
- If the event happened, and Theia passed the Roche Limit (because it did crash into earth), then would it have lead to the formation of rings of the Earth?
The Roche limit did come into play after the collision. The Moon had to have formed outside the Roche limit from the debris field left by the giant impact. There is an even more significant limit regarding where our Moon could have formed, and that is the geostationary altitude at the time of the Moon's formation. Tidal forces make objects orbiting closer to geostationary altitude migrate inward, but make objects orbiting beyond to geostationary altitude migrate outward. Our Moon wouldn't exist had it formed at less than geostationary altitude.
Geostationary altitude at the time that the Moon formed would have been much closer to Earth than today, perhaps only ~10600 km from the center of the Earth if the Earth was rotating at one revolution every three hours, or ~16800 km if the Earth was rotating at one revolution every six hours. (That three to six hours range represents estimates for how fast the Earth just after the giant impact.)
If the giant impact hypothesis is at all correct, a debris disk ("ring system") would have formed shortly after the collision, where "shortly" means a few decades, maybe a few centuries.
- If so, would we expect to still see at least some remnants today?
No. The nascent Moon would have grabbed much of the debris disk. Some of what was left would have been ejected quickly from the Earth-Moon system, some would have quickly been driven back to the Earth, and what little remained would have been eventually ejected from the system, decayed back to the Earth, or crashed into the Moon. 4.4 billion years is a long, long time.