yes
yes, you have to replace one of the team already in the game when you want your created teams into dynasty mode. it's pretty much like the old CAS in this regard.
yes, this part is different than the CAS of the past. the custom conference that will be in NCAA can allow you to completely change up the conferences however you like. as long as the number of teams per conference remain the same as least.
no, created teams(whether you use their roster or not) will not update if you downloaded a named roster file. created teams are not part of the roster file, so you would have to do that part yourself.