To be fair, those things are easier for you to understand in that order maybe, but some people can easily understand the covalent bonding math of chemistry while being utterly helpless in mechanical engineering. It all depends on your particular strengths and weaknesses in thinking habits IRL.
That being said, the other questions are easier to answer.
The desc_key is only providing a key name for the localization file line for said key, which is the text that pops up when you hover over the lock icon in the progression menu in game.
Below is an example of some xpath i used in Trapped in Purgatory overhaul to adjust the costs of all attributes and perks to scale up as you went further into the tree.
<!-- at this rate it costs 82 perk points to max one attribute. L2 1pts, L3 2pts, L4 3pts, L5 4pts, L6 6pts, L7 9pts, L8 12pts, L9 17pts, L10 23pts -->
<!-- <set xpath="/progression/attributes/@max_level">100</set> -->
<set xpath="/progression/attributes/@base_skill_point_cost">1</set>
<set xpath="/progression/attributes/@cost_multiplier_per_level">1.37</set>
<set xpath="/progression/perks/@base_skill_point_cost">1</set>
<set xpath="/progression/perks/@cost_multiplier_per_level">1.81</set>
<!-- at this rate it costs 63 perk points to max one 5 level perk. L1 2pts, L2 4pts, L3 8pts, L4 16pts, L5 33pts -->
I'm pretty sure the math on that last comment is wrong, i probably changed the value and forgot to update the comment. But it gets the idea across. I used that to set things globally across all attributes and perks, but there is a better way from what you've described wanting to do.
Now from there we can extrapolate that attribute i'm setting to get that to happen. cost_multiplier_per_level. That can be added to any progression to adjust that specific attribute or perk's cost as you level it up. Same thing for the base_skill_point_cost attribute.
How that might look would be something like this:
<attribute name="attPerception" name_key="attPerceptionName" desc_key="attPerceptionDesc" icon="ui_game_symbol_stealth" min_level="1" max_level="10" base_skill_point_cost="1" cost_multiplier_per_level="1.5">
Or for a perk:
<perk name="perkTurrets" parent="skillIntellectCombat" name_key="perkTurretsName" desc_key="perkTurretsDesc" icon="ui_game_symbol_electric_turret" min_level="1" max_level="6" base_skill_point_cost="2" cost_multiplier_per_level="1.25">
I hope that's helpful.