You are grammatically correct in your assessment of #1 and #2, except that native speakers often apply the static in/on to active into/onto situations, and since the cases of the dog already in the river and the fellow already on the floor are not very likely, a native listener would interpret both The dog jumped in/into the river and Thomas fell on/onto the floor as instances where the dog is still onshore and Thomas is still aloft. A fortiori, since there is no chance whatsoever of her 'getting' after already having entered the vehicle, the only interpretation for She got into/in Fred's car is that she was outside on the sidewalk at the outset.
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