It's not so unusual for a guinea pig to lose teeth. The pig could have been dropped and injured its teeth, or it could have done it by biting the cage bars. Guinea pig teeth grow very rapidly, and unless the roots are so badly damaged that they won't regrow at all, which isn't at all likely, they should grow back.
However, you may need to hand feed until the teeth grow enough for the pig to pick up food and manipulate back to the molars. Cutting things into matchstick-sized slivers will help, and you may have to push the back into the back of the mouth. You should also make a pellet slurry for the pig to eat -- it MUST eat or it will die. See
https://www.guineapigcages.com/forum/threads/115000-Handfeeding-a-guinea-pig.
I'd try an antifungal cream behind the ear. Use it twice a day for a few days and see if you see any improvement. If not, switch to an antibacterial cream. With either cream, apply sparingly with a q-tip and rub it in well so the pig can't ingest any.
We've seen a fair number of guinea pig toes that look like that one. Sometimes the area is infected, sometimes not. I would soak it in a dilute epsom salts solution twice a day for a few days. But I'd also keep an eagle eye on it, and if it develops any signs of infection -- heat, redness, odor, swelling -- I'd see an exotic vet.
Be very sure not to let the pig ingest the epsom salts solution or get any in its eyes. You'll have to rinse the foot well after the soak. An upside-down bottle cap makes a good GP foot bath.