I do not think the nuance you listed gives the full picture.
This can be said if you bought the bag before you returned (i.e., on the airplane, in the departure airport, etc.).
This can be said if you bought the bag before you returned, or when you completed returning.
So, the natural reading of the second sentence is probably that the bag was bought when you returned, but it's rather ambiguous and could easily mean before you returned.
There is a more important difference between the sentences though, which is that the former sentence does not necessarily imply that you got home:
"When I was returning to my country, I bought a bag. But, it turns out that I never actually got home."
Semantically invalid, 帰った conflicts with 帰りませんでした.
As mentioned, your first sentence does not necessarily imply that you got home, which is why you can add でもやっぱり帰りませんでした.
Note, that behavior of the plain form in relative clauses only works like that for a state-change verb (e.g., 帰る), not for action verbs (e.g., 走る).
"Back when I was running the streets (every day), I saw an alien."
When your embedded verb (走る) is an action verb in plain form, if the matrix predicate (ある) is past-tense, the embedded verb is forced to take on a habitual reading.
In this case adding でもやっぱり走りませんでした, "but I never ran" does not make sense semantically.