Gonna have to skip over some stuff, or just touch on them very briefly here, because when you get into response cycles like this, they tend to snowball as each response becomes more detailed than that last. If there's anything specific that I miss, that you want me to address, just point it out.
20 minutes for a mission isn't very unlikely at all. I dunno, maybe I should just create a new character and do my best impression of some first time play to get a time on how long the missions take me. The missions are on average
very short though, that I know for certain.
The darkness zone's aren't equivalent to a Halo level at all. That's nonsense. Go pick ANY darkness zone in Destiny, and compare it to a Halo level... any. I understand that it generally makes sense to believe a dev over a player, but Bungie's Molyneux-esque claims for Destiny have been shown to be hilariously inaccurate plenty of times. They had a game to sell. They initially claimed that each planet was going to larger than the entirety of Halo: Reach. Their word isn't worth much here. That par time video shows, that Halo levels are lengthy not purely as a result of your progress being blocked off. That run contains pretty much no situations where the player becomes trapped for an extended time window in order to pad the level's length out. You say it's just like Destiny? Watch this...
https://www.youtube.com/watch?v=8Z-6PG3hKak
This is what happens when you approach a Destiny level in a similar fashion. At 40 seconds the player gains control, at 3mins 30 they've reached their final destination "a monster room... surprise!", and then they remain in that room fighting waves of enemies until 6mins 10. End level. Now, this player is overlevelled, however the area of the run that would have impacted most is actually just the end, as that's where your progression would be halted... so he would have just spent even longer fighting those waves of enemies there. And besides, the game being artificially lengthen by the levelling rather than content is my argument rather than yours anyway.
I was wrong about the drop-off point though, I'll give you that. They do however tend to drop you off someone other than where the mission takes place, so the first thing you'll often do is immediately summon a sparrow the moment you gain control.
Buzzy Beetles are from the original Super Mario Bros on NES btw. They replace Goombas on your subsquent runs after beating the game. Basicallty extending the playtime despite not offering new content.