I don't have an Eheim canister but I'll try to answer your questions generally.
1. Always unplug the filter before doing any maintenance. Close the valves on both the intake and outtake lines. I learned the hard way that even with the power off, gravity will make the water pour out of the filter when you open it, so make sure to close the valves.
2. What was provided with the filter sounds good. I don't have an Eheim canister but I use some of the Eheim media. Your sponges may look cruddy and discolored, but unless they are totally falling apart, they will be fine. I would remove the carbon. I don't run carbon all the time. I haven't run it yet in my canister at all. Only run carbon if it's needed - to remove odors or medicine. (If you need it to remove odors more than just occassionally, you may have another issue.) Can you put some filter floss in the filter without removing the sponges? I put some floss on top of my topmost sponge. This layer of filter floss is a great water polisher.
3. If the moss or other debris is not passed around the impeller, you may not have to worry about it clogging up the impeller. My impeller is in the top housing (the top that you pop off for maintenance). However, I did find a good bit of my Eco-complete substrate at the bottom of my canister when I did a cleaning. Eco-complete does have some fine silt in it. I wasn't too pleased to see that in my canister. Here's what I did: I got a plastic fruit bowl from the grocery store. It was a clear plastic shallow bowl. After eating the fruit
I rinsed and cut the bowl. Picture one-fourth of a pie shape. Put the corner of the cut edge into the corner of your tank. Put the intake tube down into the bowl. You'll have to scoop out some substrate to settle the bowl piece into place in the corner. Then pile up your substrate around the edge of the bowl to hold it in place. This may or may not help with java moss getting into the filter, but sand should not be able to get up the intake tube anymore.
4. I have been taking apart my canister filter once a month. I rinse the pads in the removed tank water. I don't rinse all the pads at the same time. I also have some Eheim media in a filter bag. I rinse the whole bag - I don't take the bag apart. I do throw away the old filter floss and put new floss in. Since I don't have an Eheim, I don't know what you mean by using the screwdriver in the lid.
5. Not sure what you mean...you can have a rush of water get sucked out of the tank through gravity from the intake pipe if you don't close the valves before opening the canister. That would get messy!
Hope this helped some!