Any word can become a slur if hatred is the intent behind the speech. If you are simply talking about gay women in the LGBT community, then I don't see anything that would make lesbian a slur. If you're spewing vitriol at a group and using lesbian in a derogatory manner then is will likely be perceived as a slur.
No, lesbian is not a slur a against Lesbos. Lesbian is a category for homosexual females while Lesbos is a Greek island off the coast of Turkey. Lesbian and Lesbos have nothing in common other than sounding similar. If lesbian qualifies as a slur against citizens of Lesbos simply for sounding simiar, than one would also have to include words like lame a**.
No, I do not think that the term lesbian is an insult to the citizens of Lesbos. It is just a word, and people need to stop being so politically correct about everything. There is a good reason why the term emerged from this location, but I don't think that anyone really associates it with that anymore.
No, lesbian is not a slur against the citizens of lesbos, unless the citizens are hurt by it. If this is what they choose to call themselves, then we should simply go along with it. Not everyone thinks of "lesbian" as a bad word, and therefore they can choose the name for them.