Welcome to cdf. I thought about your posting for about a half hour before responding. My suspicion is that all the quotes you have posted are irrelevant to your problem.
Have you set CloneDVD to "DL +/-R" on the pulldown menu next to the compression meter?
You said "13 unsuccessful burns" but you have not said in what way they are unsuccessful ... did you get an error message? ... does it not play in your player? ... did cloneDVD not go to the burn phase? ...?
What brand and model DVD Writer drive are you using to burn DL?
Are you using Verbatim brand DL +R media or some other brand? (I suspect this is the most probable cause of your issue.) At this point in time, only Verbatim manufactures reliable double layer +R media.
And also, (for player compatibility) don't finalize your burns to the +R standard ... instead, be sure to booktype (bitset) your backups to the DVD-ROM standard. This is a function of your burner, it's firmware or a utility program that might have come with your burner or a third party utility such as Nero's "CD/DVD Speed" utility.
In addition and in case you were not aware: CloneDVD is a transcoder software, not a 1:1 duplicator. CloneDVD will remove the original layer break (normal behavior) and place it at the end of the chapter closest to the original's layer break (normal). Double layer backups made with CloneDVD will play fine in any reasonably modern DVD player.
If you want 1:1 duplicates of your originals (including preservation of the original's layer break), then use CloneCD instead of CloneDVD