While I can't answer your specific questions, I can offer some advice based on my experience.
If you have only one EHD dedicated to both a clone and Time Machine, you risk losing both backups if the drive fails. Personally, I have four EHDs, one dedicated to TM and the other three as clones (I use Carbon Copy Cloner but that probably doesn't make a difference compared to Super Duper).
My 2 TB TM drive (Hitachi HDD connected via Thunderbolt 2) will become full about every two years. At that point, it becomes so corrupted that even Disk Warrior can't fix it. My only recourse has been to erase the drive, losing all data, and redo it. Because I have three clones, that's not a big deal.
So, my advice is to have separate EHDs and be prepared to lose data eventually. Some people keep the old TM drive in storage (in case of emergency) and simply start a new one.