This is correct. the only DL aircraft that had an "oxygen tank" in recent years (instead of a chemical system) was the 747 if I recall correctly. even then the oxygen wasn't recirculated.

That can happen. FAs are trained to open the panels manually, but generally we wouldn't have the opportunity because before we even get to that point the aircraft is usually descended to a safe altitude. our the pilots are trained to descend rapidly, we take our seats or secure ourselves in place immediately at DL. some airlines are trained to monkey bar (move from mask to mask down the aisle) but we don't do that at DL.