It looks like he didn't live tweet the insertion operation so much as the cleanup operation (backup helicopter, destruction of helicopter, etc.) The explosion he heard was almost certainly the destruction of the downed helicopter, which would have occurred after the backup helicopter had arrived. The backup helicopters were seemingly of a different type.

http://tweetlibrary.com/damon/osamaraidlivetweets

Stealth is all relative. There's no stealth technology that renders you completely undetectable, just technologies that decrease the effective range of particular forms of detection.

For example, if a normal Blackhawk can be heard approaching from a distance of x miles, while one of these modified helicopters can only be heard from a distance of x-c miles, then the enemy has that much less warning to respond to your approach.

More likely Radar and IR stealth than silenced. It was the trip in from Afghanistan they concealed. By the time they were over Abbottabad it was too late to do anything about it.

Stealth jets are generally quiet on the approach and noisy as hell once leaving as they direct the vast majority of the sound rear-wards. Once they've dropped their bomb or showed up on the scene there's no hiding them, so why try?

It's eerie how quietly a stealth jet can creep up, and crazy how loud they are when going away, an effect obvious at air-shows with stealth-modified aircraft.