Richard,
Some of your questions need a personal inspection to answer, however I will try to give you some guidance. The roller cap is self explanatory, buy a new one. The bar above the door is usually damaged by a pothole or rough road while driving with the door closed. Normally the round tube does not bend, but the cast metal connector on one end of it will break, and let the bottom edge of the door fall to the bottom of the shower. However if the tube above your door is bent (I can not tell from the picture), there is only one thing that can be done, replace it. I would also replace the connectors on both ends. One word of caution, replacing the top tube requires disassembling at least half of the shower, if not all of it. If the tube has been replaced before or if you have a small angle drill you can remove only one wall to replace the tube, otherwise the entire shower enclosure must be removed.
If the lower retainer is worn or damaged, replace both of them. The gap at the lower edge of the door looks like the edge has been damaged at some time, but that should not be a problem because there is a wiper inside of that area to seal the area between the door and wall. If your shower does not have that wiper, install one.
When ordering any parts, make sure that you are ordering the right ones, because most of the Cesana parts pictures show the shower upside down from the way that it is installed in a Beaver. So in their parts diagram the tube for the top of your door may be on the bottom. The key is to look for the long and short walls to determine if their diagram is installed the same way that your shower is.
Gerald