Floor rot by the door is not a big problem. You just have to replace a separate piece of plywood in the walk way. The biggest issue will be dealing with the screen door if you have one.The lower floor sections are calked between the wood and steel and and the plywood is screwed in place. You should put resin on the bottom of the plywood. The only other thing to do is put the aluminum extruded cap on the edge of the plywood. I use a router to get the wood to the proper thickness. 

Door sag: Check your frame radius for cracks. The walkway has two metal pieces going from the frame almost to the shell. There is a piece of plywood which is glassed over holding the metal to the shell. Often the wood rots out. You will have to cut out the old glass and wood. Realign your door frame and shell. install a new piece of plywood and glass things back together. It is not going to be easy as the floor repair.

Good Luck

Eddie