Is it acceptable to email the authors of a paper for the slides they used while presenting it in the conference?
It's never a problem to ask. In particular if you are interested in extending their work, they should be interested in providing you with more information. Perhaps the extension will happen in collaboration with them, but at the very least you're going to cite their paper. In my experience, views on sharing slides vary, but the objections I've heard related to unpublished material; I understand that in your situation, the material is already published.
I would explain why. It's polite to do so. But do keep the mail short. You could write that if they're interested, you can provide more details on your ideas; like that, you don't swamp them with unrequired information, yet you're open from your side in stating your ideas. Such an offer shows that you trust them, and as they need some trust in you if they share their slides, that is probably a good idea.
I don't think they would feel offended. 
Depends on the field. If there is no clear first or corresponding author, I would contact either one of them. They would probably either reply or forward the e-mail to the other author.