I really like that interpretation.
So, Watson looks out into his civilian life and doesn't see enough action, or he doesn't experience enough action (MC Problem: Inaction), but instead of going out and finding it, he internalizes the problem (Approach Be-er), and it manifests as a limp and tremors, which he attributes to his PTSD (MC Focus/Symptom: Reduction). Running around like a couple nutters with Sherlock alleviates or addresses his symptoms (MC Direction/ Response: Production), but it's not until he takes initiative and brings the case to a close through his own actions that he truly resolves his issues (MC Solution: Protection).
Then he and Sherlock strut off into the night, presumably to carry on protecting the people of London.
I think there are some storytelling issues with this throughline that don't serve the audience very well, but I reckon that doesn't count against it as far as Dramatica is concerned. It looks like the storypoints are there even if Mycroft spoiled the ending a bit.
We discussed the author's proof a bit a few days ago, and I wanted to add that in addition to Sherlock's realization about Watson and his change, we also get to see it in Watson. When Sherlock confronts him about it and points out that Watson is handling things well considering he just killed a man, Watson just sort of shrugs it off: "I did, didn't I? Well, he wasn't a very good man."
Do we see Inaction as the problem in the OS? Is that the victims went along with Cabbie's game?
I can better see Protection as the Solution and work backward. If the victims had protected themselves, they might have gotten away because the gun was a fake. At the very least it would have left physical evidence behind, making it clear to the police that these were murders. Their Inaction makes it look like suicide, which allows the Cabbie to continue operating.
The Pink Lady doesn't just sit there and die. She protects her interests, insofar as catching her killer are her interests, by planting her gps-enabled phone on Cabbie and scratching the password into the hardwood. This ultimately gets the Cabbie caught.