So poking around the forums I found out that, like clerics, paladins don't have to follow a specific deity and can simply be divine agents of a given ethos.
Can a paladin be such a godless sort in terms of his power source, and still count as "worshipping" an actual deity (with less alignment restrictions) and qualify for, say, a Divine Fighting Technique (which is normally locked to a follower of that deity, and paladins as far as I know can't serve CG deities).
This is something that could be easily justified in-character, too. For instance, a paladin could align himself with the divine concept of travel, luck, goodness, or liberation (none of which is inherently chaotic) and still revere Desna despite not directly serving as her agent.
I would say no in this case. A paladin must keep all commitments, vows, oaths etc they make. A chaotic deity with Desna's portfolio would likely have some rules or bits of ideology that would violate the lawful portions of the paladin's code, and pledging service to Desna and then not being true to Desna's ideals would also violate the paladin's code.
If you were asking about Erastil or Iomedea I could see it working.
I don’t think that a character who worships a idea can still worship a deity. By worshiping something you are choosing to put that first above all else. If you worship a deity you put that deity above anything else. If you worship an idea that idea becomes your highest priority. Even when the idea and deity align perfectly you have to choose one above the other. A worshiper of an idea may honor a deity as being a servant of the idea he worships, but if a conflict ever comes up the worshiper of the idea will always stay true to the idea.
In all honesty I think the whole concepts of worshiping an idea was put in to appease those that find worshiping a deity objectionable. Also note that this is an optional rule, and not all settings permit this. If you are playing in Golarion and following the official guidelines all divine casters must have a deity. Your GM is of course free to change that, but at that point it becomes a house rule. Since Desna is a Golarion deity by default what you want does not work without a house rule.
A paladin gaining their powers from Law and Good is a different mechanical area than clerics gaining their powers from the same.
Unless your setting is Golarion, paladins do not gain their powers from a deity unless they explicitly choose to. So assuming you don't want to undertake a god's paladin code but still worship them, you could and still qualify for feats and divine fighting techniques. Because a paladin doesn't "Worship" Law and Good, they are exemplars of that concept.
Of course, any deity that enforces an ethos that doesn't line up with the paladin code is a no go.
To expand on what I said earlier, as I didn't have time when writing it...a Paladin could in theory derive power from the universal concepts of law and good and worship a deity. Just not a deity that wouldn't be able to have Paladins in the first place. The concepts of being lawful good would interfere at some point with a deity's tenets unless they were LN, LG, or NG (deities normally capable of having paladins).
As to respecting or revering things which lie in a deities domain that isn't lawful good...just because you revere the concept of travel and you want to personally ensure that the roads are free of bandits (what a paladin like thing to do!) doesn't mean that you're a worshiper of Desna. You can have interest that align with deities, without being aligned enough to really be a worshiper.