1. That sin we commit right before we die .
2. I believe it is inline with all Paul's warnings to us Christians. Paul never singled out Christian X as saved and Christian Y as unsaved. Paul was being respectful. That is why we will forever debate OSAS . A human cannot see the depths of a heart, only works. But God knows who is His and who isn't. So the sin unto death is clearly rejection of Him. Rejection of Him is full measure when our sin is full measure. The Amorites were not destroyed because their sin was not full measure Gen 15:16. Sodom was because their sin was full measure.
Ananias and Sapphira should be compared with Judas. A single action that reflects a state of the heart being so lost / a depth of evil intent that causes our love of sin / evil to be '''full measure'''.
I can not agree with that.
You are saying that Ananias & Sapphira were not believers. That is not the case IMO.