Activate "FAF" leg

What specific approach (with transition or vectors)?

What was the IFD active waypoint before trying to Activate Leg to the FAF?

What IFD waypoint was highlighted on the FMS - FPL tab?

Were there any gaps in the flight plan prior to the FAF?

Just to confirm, there is not an activate FAF leg LSK. In order to activate that leg in this scenario, one would have to use Activate Leg by effectively doing the following:

1) Verify that BADDA and BINGG are indeed connected without gaps in the FMS FPL tab. If the approach was selected as a non-vectors approach from the procedures database as described, this should not be in question. 

2) Verify that BINGG is the highlighted cursor waypoint on the FMS-FPL tab. If not, make it the highlighted waypoint. In your scenario, I can imagine that EMRSN may have still been the highlighted waypoint as you said you had last performed a direct-to EMRSN. If EMRSN was both the active waypoint and the cursor highlighted waypoint on your FMS-FPL, then the Activate Leg LSK would not have been available as an LSK option.

3) If conditions one and two are satisfied, there should be an Activate Leg option at LSK 1.

OK...I ran another several IFD Trainer app simulations completely in heading select as you described and think I may have your answer. Ironically, the possible answer was actually mentioned in your description of the event. My theory is that as you were vectored on this approach past EMRSN the IFD was auto-sequencing your active leg based on your vectoring. By the time you were on your intercept heading and wanted to to manually update the active leg to BINGG, the IFD had already beat you to it by auto-sequencing to the BADDA to BINGG leg...only in the heat of battle, you didn't realize it. That's why you couldn't get an Activate Leg LSK prompt. The leg from BADDA to BINGG was active and BINGG was your already the IFD active waypoint. 

There is still one piece that doesn't fit. If the BADDA to BINGG leg was auto-sequenced to be active, and subsequently GODLY to BADDA was manually sequenced to be the active leg, I could not get the IFD Trainer app to auto-sequence back to the BADDA to BINGG leg. At this point; however, the BADDA to BINGG leg could be manually activated using the Activate Leg LSK.

While the IFD PG doesn't go into much detail about the mechanics of waypoint auto-sequencing, it appears the trigger to auto-sequence to the next leg appears to be passing either over or flight plan track abeam the active waypoint. It appears that if BADDA to BINGG was already auto-sequenced to be active, then the IFD would be passed the the point to trigger that leg again when the IFD was subsequently manually sequenced to an GODLY to BADDA leg.