Thanks for reading and for your questions. Actually for me to be able to do justice to your questions, I think I need to make a post about tooth eruption and cavity formation that will adequately address it all. Meanwhile, let me try and answer some of the issues you raised.
cavity formation involve an interplay between 4 factors : bacterial, susceptible tooth surface, sugar and time. the whole process involves periods of demineralization and remineralization depending on the PH of the saliva at each time. however when demineralization progresses faster than the rate at which remineralization occurs ,cavity formation will occur and it is an irreversible process.so it can never grow back. Also people have a lot of cavity on their teeth because of poor oral hygiene, consumption of lots of refined sugar containing diets. I will explain more in my post .
To the other question, there are two set of dentition that an individual will have in life, primary(milk teeth) and seconday dentition(permanent teeth ). primary @early stage of life and secondary later(after 13yrs). so if it is primary that is removed, the permanent will replace it but if anything happens to the permanet, then no chance of regrowing again.
Any toothpaste that contain fluoride is okay to use .there are so many of them in Nigeria... Close up, Oral B etc.
I hope l have been able to answer your questions, any grey areas will be elucidated more in my next posts. Thank you.