Sadly, Covenant-designer man doesn't really innovate. ...there is a VTOL craft later, though. So, you eventually get your wish!

The door opening wrong is also a clever way to gate your banshees while still re-using the level.

The moody atmospheric music is very good, and it's one of my favorite tracks. If for no other reason that is scores a very poignant scene in the anime.

Pretty sure Keyes unleashed the flood by breaking into places and releasing seals - we saw in the cutscene opening locked doors and such. Though not just Keyes; the Covenant were noted as trying to break in for the 'weapon' as well.

Being 100% honest... in this area I just Ghost-ed past all the enemies, and jumped a Banshee after the 2nd or 3rd time we died. Probably wouldn't work on Heroic, though.
...I died more times to the Sentinels guarding the door than I'd care to admit.