What do you think happened?

You are correct. Benson suggested Jessica did not survive the firing squad since she appeared to Benson as a ghost.

I believe but not 100% sure that Jessica even told Benson something to the effect of "I was about to be shot in a firing squad and the next thing I know, I am here".

Ahhh, no.

While I thought the third season was just as good as the first two,I found the final season to be most frustrating.Benson left for his own show at the start of season 3,and Corinne split at the beginning of season 4,due to Diana Canova getting a series of her own as a totally different character.These losses,coupled with more writers each year,led to a "too many cooks" kind of situation.The early episodes were solely devised with superb skill by the extraordinary writing of Susan Harris,but the later seasons seemed to get away from the original concept.That final year saw all our favorite characters become shockingly unlikable,especially the very loving relationship between Burt and Mary,which was so vital to the show's longevity,and given the warm regard the two actors had for each other.After hearing them both proclaim their undying love for three seasons,it was a huge disappointment to see their relationship fall apart in the closing entries.Mary had earlier described a woman whose husband was always working,who has Cocoa Puffs and vodka for breakfast each morning,and now Mary herself is reduced to being an alcoholic with an alien baby,a storyline that went nowhere.Jessica's affair with El Puerco simply left me cold,and it lasted the whole year.For me,only Danny remained consistently interesting throughout the entire run; however,once they killed off his wife Elaine(the only character other than Peter Campbell treated in such a fashion),it seemed that all of his storylines lasted about ten episodes apiece.Chuck and Bob remained exactly the same from beginning to end,as did The Major,unchanging but indispensible.Jodie remained a tricky character to write for,but generally remained interesting,until the very end,when he winds up an old jew(perhaps in homage to his lost friend Barney Gerber,played by Harold Gould,who never reappeared after episode 12).Eunice remained one of the least interesting characters,while Billy,who had decent storylines for three seasons,became a general in El Puerco's army and virtually disappeared as a viable,intriguing character.Jessica certainly held up her end of things,but it was an uphill battle,while no other character went through so many changes as Chester,who emerged unscathed in the end(though it was just a lazy cop-out to have him be the killer of Peter Campbell,since he never knew Peter,who recognized his killer before he died).One final problem was that the new characters that were introduced the last year simply weren't strong enough to keep audiences from losing interest,especially the boring revolutionaries.Benson's replacement,Saunders,as played by the distinguished Roscoe Lee Browne,was by necessity a very different character than his predecessor,but a solid addition that could have been disastrous.Solid pros like Dick Miller,Richard Libertini,Nita Talbot,Doris Roberts,Jack Gilford,Lynne Moody,and Barbara Rhoades,like Harold Gould,could only have been good for the show,and it's a shame they didn't have larger roles.Still one of the most indelible comedy/drama series in television history,and it would be interesting to see how future generations react to 20th century programming.

The "ghost" said she was in a COMA, not dead.

Somehow, I see something absurd happening with Jessica, such as:

The sound of firing is the squad itself being shot by snipers. Jessica gets freed and instead of El Puerco being made leader, SHE gets made leader. El Puerco later, out of jealously, decides to off her with poison - which instead puts her in a coma (leading to the Benson scene).