Just changed the lower left tail light on my 2013 XC70 and your tutorial helped a lot. Was able to get to the 10 mm nuts off by just removing the carpet side access panel and the triangular shaped piece of Styrofoam in front of the taillight. Used a 1/4 inch deep well socket from Harbor Freight ...

If the vacuum line fix doesn't work you might try replacing the oxygen sensor.
My understanding is that the forward sensor controls the fuel air mixture so if you are going for fuel economy you only need to replace the forward sensor.
I wouldn't worry about replacing the rear one. My Volvo expert ...

Just replaced the struts and shocks in my 2002 v70 (2wd non turbo) at 82,000 miles. After talking with my Volvo expert decided to go with the OEM Sachs and am extremely satisfied with their performance. Huge difference in the ride and handling. Ordered all the parts from FCP Groton don't forgot in ...