These are great questions but I have my reasons for focusing on LTC. And the computer is already on running two 290s....so I might as well try and get 100+ hash out of the proc.
i don't think it's worth it. I was mining Prime coin on my cpus and at full cpu load my gpus suffered about 20kash each.
I would say its definitely not worth mining ltc with a cpu. You might come out ahead with a cpu oriented coin like prime coin and then just sell them for ltc. Personally i didn't feel like the extra pc load was worth the couple extra $$