You have unlimited ammo because it's a dream! The last part (after being hit by a laser) is a dream. Where did the plants come from when you get up? I mean if you look behind you there's a bunch of shrubbery that wasn't there before

I figured out why the Reaper doesn't shoot at the Normandy when it was picking up Garrus and Liara!
Because the people on the ground are a far bigger threat than the Normandy is. If a single person reaches the beam, it's game over for the Reapers. The Normandy probably can't even do significant damage to the Reaper, while the soldiers on the ground can now have the ability to stop every Reaper, but only if they make it to the beam. Which one does it make more sense to focus shooting at?

But then why can't the Normandy fly close and drop someone off next to the beem

There's a giant space battle going on and the Normandy is one of the most advanced ships out there.

Could have all ended the whole fight by dropping someone off right there...

The Reaper would just shoot the Normandy if they dropped troops right on top of the beam, and the gauntlet doesn't give them much better odds.

The Normandy has gone through worse, and besides it would have distracted the Reaper

Hey we gotta win one way for sure, and that works.... Rather that than win in a dream...