Yeah alrighty let's run through these questions:
Dmiri + 4 bandits are visible
The platforms at I14 and T16 are 10' off the ground with a visible rope ladder. Not low enough you can likely directly attack the people on it without some kind of enlargement spell but not ridiculously high up either.
https://2e.aonprd.com/Actions.aspx?ID=33
It counts as a ladder so the DC 10 is super easy but you want to roll anyway because a critical success means you can get up the entire ladder in a single move action. Otherwise a normal success means 2 actions to climb with a bit of wiggle on either side because the ladder eats up 20' of movement per 5' climbed.
The platform Dmiri is on is also 10' up off the ground and you can see the ladders on either side. There is another platform in the middle that goes up another 10'. The guard was originally located here but came down when Dmiri climbed up.
Yes you can run underneath the platform.
Yes the door to the shack is visible. I didn't go over Dmiri climbing up in her introduction, apologies for the confusion. She did not use the ladder but instead hopped up thanks to a very good climb score and using the tree at S11