I devote a lot of resources into thinking about philosophical problems which does not seem to contribute to my genetic fitness. Have I been "subverted" by a selfish meme (i.e., the one that says "the unexamined life is not worth living")?

Possibly. It depends on why you do that. The other main hypotheses are that your genetic program may just be manfunctioning in an unfamiliar environment, or that the philosophical problems do - in fact - have some chance of turning out to be adaptive.

If so, I don't feel any urge to try to self-modify away from this.

Right. So: that could be a result of the strategy of the meme to evade your memetic immune system - or the result of reduced memetic immunity as a result of immune system attacks by other memes you have previously been exposed to.

Any meme that makes a human more meme-friendly benefits itself - as well as all the other memes in the ideosphere. Consequently it tends to becomes popular - since every other meme wants to be linked to it.