Your basic approach is fine - upon your passing survivor SS income will likely be lower and expenses may go down (except taxes). In the "Other Income/Spending" tab in FIRECalc you can enter the SS reduction as "Off Chart Spending" and the spending reduction as "Pension Income (or off chart spending reduction)". Both would be inflation adjusted. You could also net them out and enter it, plus or minus, on one line.
However, I would not pick a date.
To be conservative in planning, I would assume the event happens this year. If that scenario works in FIRECalc you are good to go, so to speak. If you run that scenario and there are high failures then you might want to make some adjustments.
FI and Semi-ER March 24, 2017
Consulting to stay engaged
"All models are wrong, some are useful." - George Box