Wow and i thought I carried on with my posts your gonna give me run for my money i can tell already 
Ok i will try my best to answer some of your questions as far as the error issues you have posted they might require @markus101 to view or one of the other more knowledgable people on these forums 
(1) - Ok first of all it is always best and recommended that both NzbDrone and Sabnzbd are on the same machine this is to avoid issues of them trying to communicate with each other over the network and also it is best if they are on the same machine as your root folders (the folders where your shows are) this avoids file path issues when trying to import existing shows into NzbDrone.
(2) - You can set naming formats in NzbDrone to your liking so if you like stuff named a certain way you can customize this so anything that NzbDrone processes gets named in that format.
(3) - Yes it is best to let NzbDrone handle the moving and sort of the episodes this is one of it’s greatest functions it will save you time and effort in manually moving and renaming each episode.
(4) - There are things in place for NzbDrone and Sabnzbd to handle things like passworded or incomplete downloads these are not foolproof but are getting better and better as things progress NzbDrone handles PROPERS/REPACKS much better than Sickbeard whereas Sickbeard would only check once per day and the stupid and inconvenient time of 1AM NzbDrone is constantly monitoring for such things so for example you have an episode that is downloaded then 20mins later a Repack or Proper is released on the Next RSS Sync it will detect a newer release and then download that one and replace the nuked release with the PROPER/REPACK and do this automatically.
(5) - NzbDrone does have backlogging capability but only if you indexer supports it. how it works is next to each episode and each season there is things called flags those are the little black… well flag shaped things next to each one when this is showing as black that means NzbDrone is set to watch this episode which means if you have your cutoffs set and it has not met them with this particular episode/season/etc… it will upon detection of a higher quality file within the parameters of your cutoff download that and in the same process as PROPERS/REPACKS replace the previous file with the newer and better file.
(6) - Regarding performing backlog search yes you can do what you say and manually click each season invidiually (The little Magnifying glass) or if you want the entire series (all seasons) then there is another Magnifying glass on the black banner where the show title is displayed
(7) It is recommended to let the Completed Download Handling function work if using Sabnzbd it works well and is easy to configure simply turn it on in NzbDrone you will however have to create a separate folder than the one being used for the Drone Factory for the downloads to temporarily sit in before being processed you will also need to set a category in Sabnzbd to point to this folder
If i have left anything out let me know also you might be interested in a guide i created it is a work in progress but i tried to simplify things for beginners laying out the main functions and how to configure both Sabnzbd and NzbDrone to work
as far as convincing goes it is simple Just look at the quality of the software and check out these forums and see the community support there if that doesn’t help to convice you i am not sure what will 
best of luck and welcome
here is a link.