I have recently been reading about the formation and early evolution of the Solar System, and how the outer Solar System very likely did not form in its current configuration and has almost certainly undergone some reshuffling and migration, and possibly ejections of ice giants that are no longer with us. However, the inner System seems to be a little more glossed over, even though it has its own set of problems, for instance, the origins and properties of Theia, the hypothesised impactor that led to the formation of the Moon, and Mars' low mass. Simulations seem to suggest a 0.5-1 Earth mass planet should have formed in Mars' position, and several explanations have been proposed to resolve this discrepancy, such as the Grand Tack, a hypothesised migration route Jupiter took in its infancy, before Saturn stabilised it and migrated out again. However, there are also problems with this explanation. This has led me to the question.
What if the inner planets, like the outer, also did not form in situ and have migrated and swapped positions? I have seen some scenarios in which Neptune and/or a third ice giant formed between Jupiter and Saturn, the sandwiching between two gas giants leading to a deficiency in material. What if in the same vein, Mars actually formed between Venus and Earth, ending up a low mass object due to the deficiency in accretion material, and was later pushed into its modern orbit by Earth. In fact, we could perhaps take this further: the faint young Sun paradox (I'm aware other solutions exist). Maybe the forming configuration was Mercury - Earth - Mars - Venus, with a wave of instability stemming from the two larger terrestrials perturbing Mars leading to Mars being kicked into it's current position, with the resulting instability of the larger terrestrials leading to them swapping positions. What implications would such a scenario have on the origins of life? If we assume said scenario removes the need for the Grand Tack, that begs the question of whether this 0.5-1 Earth mass planet did indeed form, and was later ejected during this wave of instability, perhaps by Earth or a combination of Earth and Jupiter, if we assume it was closer to 0.5 Earth masses than 1. I could go on to suggest the modern Asteroid Belt was formed by this planet and another hypothetical terrestrial that formed in the vicinity, but the catastrophic collision scenario has been largely debunked, although the removal of the GT could bring this back into question.
I haven't included Theia and the impact as that likely happened in the Earth's infancy, long before this wave of instability took place, and it has been suggested in recent times that it was in fact an outer Solar System object that had migrated into the inner Solar System. Whether the latter point is true or not, doesn't really matter to this scenario, unless it happened in the Solar System's infancy.
So, how credible is this?