1.) yes, it will overwrite any discovered fields tho (and leave custom and comment field). if the IP disappears, it will go into transient and then available (if you didnt set the subnet or the specific IP to not scan). If you want it to stay in reserved state, just disable the scanning on that single IP or the whole subnet.
2.) no, see above (you can set the amount of days it will stay in transient too, i believe 2days is standard but you can set it to a maximum of 340days)
3.) no, thats what the option to turn off scanning (edit properties of the IP or subnet) is for
Thanks for the information. I'm sure I will have more questions as i get more familiar with the product.