If you remove the user and will be an empty string instead of "User deleted" - the site is quite well-designed. These things need to finish in the course of operation: a) if empty result, then write it, and if not remove and add a new field 1, we will write "User is banned" (all of a sudden then want that) and throughout the site to check this field for a unity and do respectively. actions, etc.
Do I need to consider that someone can get into MySQL is not necessary to consider. If someone is going to climb, the site is not only yours :)
About the failure on the host when adding - it is almost impossible.
Even if the user is not 1 table, and then logging will occur for 0.01 sec. or faster.
A chance that the entry in table 1 would happen, and to another in no time - very, very small :)
Although under certain conditions this could be, for example, when loading large volumes