Delta Force selection is originally based on SAS selection, so why is there no brutal jungle phase for Delta Force? It seems like it's based only on the Brecon Beacons section.

The jungle is constantly trying to fuck you up. Everything wants to eat you or lay eggs in you. Many of the trees are covered in 2" long spikes so you can't even put your hands on them (Escoba, often referred to as bastard trees, as you will regularly hear a hushed voice in the dark exclaiming ‘Bastard!’ when someone inadvertently grabs one).

Your clothes are rotting off your body. It is hot so you sweat, but it doesn't evaporate due to high humidity so you can't cool down. You just sweat more and dehydrate. Visability is a few meters which adds a degree of claustrophobia, and navigation is very hard. Your fundamental military skills need to be top notch to function well.

CAG in the jungle

Do you think it is likely that Maegor was presented a young dragon at some point, almost to the point of full-bonding, only to scorn it for Balerion in the end? If so, which one could it have been?

Jungle phase (properly known as SOP and Tactical Training Phase, or just TTP) isn't really so much about the jungle. The purpose of it is to assess the candidates ability to work in small teams in an austere environment. Their individual self discipline, basic military skills, adaptability, team work, robustness and determination when under constant and increasing pressure and while being closely assessed by the training team are the key parts. The jungle just happens to be the perfect environment for this.

Bastard tree

All that being said, CAG do actually carry out jungle training, however as far as I know it just isn't part of the recruitment and selection process/OTC

Miley Cyrus Recalls How She Used to 'Hide' Drug Spending From Her Accountant - TooFab

CAG (‘Delta’) have different requirements for their shooters. While ostensibly similar, the mission set is not identical. Due to there being fewer units, UKSF traditionally do things that would be the remit of other US SOF units. CAG are able to focus on the counter terrorist aspect of the roles in the Operator Training Course (OTC - basically their version of UKSF continuation training). They look for similar traits as mentioned above, however the pressure is applied in a different environment and one more applicable to the units tasks.