1. The multiplier takes the base cost and multiplies by the multiplier for each level, and then truncates the decimal (removes it). So the reason you got 2, 6, 15, 39 was because 1 * 2.5 = 2.5 (truncated to 2), 2.5 * 2.5 = 6.25 (truncated to 6), 6.25 * 2.5 = 15.625 (truncated to 15) etc. So you can only fiddle with the base cost and the multiplier to make the cost increase (or decrease, really) as you level the skill.
2. Not sure on that one, but I'd guess that it limits how many points you can have in the skill based on the parent attribute or something.
3. Descriptions of things go into your Localization.txt file. Look at the default 7 days Localization to understand the format. I tend to skip other languages because I'm lazy, so most of my Localization lines look kind of like this:
experienceBookSmall,items,Book,,,"Newbie's Journal",N/A,,,,,,,,,,,,,
experienceBookSmallDesc,items,Book,,,"This journal contains the experiences of a survivor of the wasteland...until they met their end, at least. Perhaps you can learn a thing or two.",N/A,,,,,,,,,,,,,
So basically, you name your desc_key something unique to your skill. Let's say your perk is called Planking (lol). I would set desc_key="perkPlankingRank1Name" (because the desc_key is actually used for the name of the rank of the Attribute or skill) and long_desc_key="perkPlankingRank1Desc" (because this is where the actual description that shows up when you mouse over it goes). Here's an example of the localization of the base Attribute for all extra skills on my server, called attExtra behind the scenes and "Continued Advancement" ingame:
attextra,progression,Attribute,,,Continued Advancement,N/A,,,,,,,,,,,,,
attExtraDesc,progression,Attribute,,,"You feel that there is more to learn, and this is the record of your continued journey towards godliness.",N/A,,,,,,,,,,,,,
attExtraName,progression,Attribute,,,Continued Advancement,N/A,,,,,,,,,,,,,
attExtraRank01Name,progression,Attribute,,,Merely Mortal,N/A,,,,,,,,,,,,,
attExtraRank02Name,progression,Attribute,,,Something More,N/A,,,,,,,,,,,,,
attExtraRank03Name,progression,Attribute,,,Enlightened,N/A,,,,,,,,,,,,,
attExtraRank04Name,progression,Attribute,,,Ascended,N/A,,,,,,,,,,,,,
attExtraRank05Name,progression,Attribute,,,Superhuman,N/A,,,,,,,,,,,,,
attExtraRank06Name,progression,Attribute,,,Heroic,N/A,,,,,,,,,,,,,
attExtraRank07Name,progression,Attribute,,,Mythical,N/A,,,,,,,,,,,,,
attExtraRank08Name,progression,Attribute,,,Angelic,N/A,,,,,,,,,,,,,
attExtraRank09Name,progression,Attribute,,,Invincible,N/A,,,,,,,,,,,,,
attExtraRank10Name,progression,Attribute,,,Godlike,N/A,,,,,,,,,,,,,
attExtraRank01Desc,progression,Attribute,,,"You are simply a mortal. You are just beginning the quest to enlightenment.\n\nLootstage +2, Max Health +25, Harvest Count and Mobility +10%",N/A,,,,,,,,,,,,,
attExtraRank02Desc,progression,Attribute,,,"You have become something more. What have you become, you ask? Who can say?\n\nLootstage +4, Max Health +50, Harvest Count and Mobility +20%",N/A,,,,,,,,,,,,,
attExtraRank03Desc,progression,Attribute,,,"You are starting to realize that there is more to existence than meets the eye.\n\nLootstage +6, Max Health +75, Harvest Count and Mobility +30%",N/A,,,,,,,,,,,,,
attExtraRank04Desc,progression,Attribute,,,"You see the air, hear the colors.\n\nLootstage +8, Max Health +100, Harvest Count and Mobility +40%",N/A,,,,,,,,,,,,,
attExtraRank05Desc,progression,Attribute,,,"You can bend reality in small ways.\n\nLootstage +10, Max Health +125, Harvest Count and Mobility +50%",N/A,,,,,,,,,,,,,
attExtraRank06Desc,progression,Attribute,,,"You are pretty sure you can feel the vibration of gravity. Heroes tremble when hearing your name.\n\nLootstage +12, Max Health +150, Harvest Count and Mobility +60%",N/A,,,,,,,,,,,,,
attExtraRank07Desc,progression,Attribute,,,"You feel that with just a little more effort, you could affect the orbit of the planet. Your strength and intellect are mythical.\n\nLootstage +14, Max Health +175, Harvest Count and Mobility +70%",N/A,,,,,,,,,,,,,
attExtraRank08Desc,progression,Attribute,,,"Certain that you have been sent with a purpose to right the universe, you have become stronger than any mortal.\n\nLootstage +16, Max Health +200, Harvest Count and Mobility +80%",N/A,,,,,,,,,,,,,
attExtraRank09Desc,progression,Attribute,,,"You have complete control over space. Now there is only the matter of time...\n\nLootstage +18, Max Health +225, Harvest Count and Mobility +90%",N/A,,,,,,,,,,,,,
attExtraRank10Desc,progression,Attribute,,,"You are the universe. Time and space are but thoughts you pondered briefly.\n\nLootstage +20, Max Health +250, Harvest Count and Mobility +100%",N/A,,,,,,,,,,,,,
Notice that the name of the very base of the attribute, attextra, is all lowercase. This is a rule for some reason - if you don't use all small letters for the name of the base attribute, it won't show up correctly. Also notice that there are quotes around the English localization on some lines. This is because some special characters can't be used unless they're in quotes. So theoretically you could enclose every entry in quotes, but it's only really necessary if you have special characters like a comma. Since all my rank descriptions have commas, they're all in quotes.