"Goodbye, Mrs. Selvig" does the favor of giving context to the uncertainty of the previous episode, which includes clearing up a few details, such as the lies to Mark about it being five months later and that their Outie counterparts had gone on to renown fame after the Innies revealed details about their treatment within Lumon.
The reality is that rather than five months, it has only been a few days, and the Outies are significantly less thrown off about the events than one would have thought. Mark and Helly are the most confused, as Mark must deal with the question of whether his sister is now trying to convince him that Innie Mark was referring to his late wife being alive rather than a newborn baby's birth. Meanwhile, Helly is mesmerized by the tapes of her Innie interacting with Mark just before everything changes. Meanwhile, Dylan just seemed happy to be brought back while Irving was spotted mysteriously at a payphone.
"Goodbye, Mrs. Selvig" is all about the Outie side of the story but also makes the interesting choice to devote more time to Helly's Outie storyline. In season one, Mark's Outie was the only one whose life was really significant, leaving the others as uncertainties. The only clue anyone got otherwise was about Helly's after her failed suicide attempt led to her Outie leaving her a fairly dark message.
The season one finale reveals that Helly's Outie is actually a part of the company, a supposed high-level executive. In fact, it is a dangerous plot twist but one that allows for a fascinating character dichotomy between Helly's two selves. Mark's biggest change is that he no longer feels the grief that his Outie was so weighed down by, and Dylan and Irving's outside personas have not been given enough development to truly determine if there are significant differences between the two counterparts.
Helly is the biggest example of a character's inner workings of their mind and how nature versus nurture may appear differently when shown through the perspective of someone who one can argue is the same person. Helly's Innie and Outie should be the same person at their core level, with the distinct difference being all about memories and experiences.
Helly's Outie knows everything about Lumon and the severance procedure. It is a part of her family's company. There are little to no moral conflicts she faces in the aftermath of participating in this procedure. In fact, the only time Helly does appear emotionally impacted is when she watches her Innie's growing dynamic with Mark, something that rubs off on her as she is seen watching Mark enter the building. But, if Helly's Innie is so disturbed and disgusted by the severance procedure without the memories of the company itself, does that mean that deep down within Outie Helly, she is also not in favor of the procedure?
Coming out of "Goodbye, Mrs. Selvig," the significance of which Helly watches her own interactions that she has no recollection of is perhaps the most fascinating aspect of the episode. Mark's refusal to accept that his late wife may be alive, as hinted to by his Innie self, is bound to be a messy storyline, especially as Innie Mark is planning a quest to find his Outie's wife.
Meanwhile, Irving and Dylan have yet to step up as being integral to the show's overall mystery, even if Irving being watched at the payphone is a significant tease of something else coming in future episodes. Still, Outie Helly participating as an executive at the company, knowing the details of project "Cold Harbor" and facing an interior battle that comes from her Innie wanting to fight against what her Outie stands for and Innie Helly having her own human experiences, is one that allows "Goodbye, Mrs. Selvig" to flourish.