"Goldie, for the explosions I assumed they could scan everywhere the plane might have disappeared - easier to see than planes & ships searching the sea for tiny bits of wreckage?"
But that depends on having recorded the explosion. If they didn't... or if the recording of the new search area no longer exists... they weren't looking there for an explosion, remember. The wreckage will still be there. Drifted, maybe, but still there.
"what if it crash-landed into dense jungle - could the canopy close over it again to a certain extent?"
It depends how it went down. If it exploded and came down like confetti or if it crashed near vertical, then yes. If it came in with a lot of forward speed, it would cut a swathe through the jungle canopy. So it's possible, yes.
"I'm puzzled by the ACARS. Weren't they significant in the early days of pinpointing where the Air France aircraft went down? Also, aren't ACARS unencrypted? If so, anyone could have picked up those signals, so why are the authorities being cagey about releasing them?"
The ACARS didn't pinpoint AF447; it just gave a sequence of failures as the aeroplane came down with a time stamp. ACARS alone can't tell you where the aircraft is. ACARS data is not encrypted but you need the right equiment to receive the signal. Aircraft have different means of communication - High Frequency radios, VHF radios, VDR, or VHF Data Radio, and Satellite Communication (SatCom). The ACARS uses all but HF to transmit data, so if it's in a phase of flight that uses SatCom, you'd need very special equipment to receive the signal.