Your lens does sound like it could be dirty! Simply wipe it down with a microfiber cloth to remove any grease or dirt that has built up. If you think it's not a cleanliness issue, check out other symptoms here. In the case where you need to replace the camera, you can find a guide on how to do that here!

Your screen sounds like it is defective! You'll have to open up the phone and take it apart if you want to be able to reach the screen - you can find a guide on how to here.

Especially with lithium ion batteries, they hold less charge each time you charge them. Although the loss is minimal, over the years, it can add up. So they will wear down due to age - simply replace it with a new one! You can find more about battery issues at this troubleshooting page.