n00b questions about using dar for archiving wikis

i'll try to be as comprehensive as possible so that i don't miss anything.

i want to make decremental backups of 7tb worth of wikis (text only)
provided by wikimedia, each wiki dump in a separate dar archive

wikidumps are updated each month. and from what i've read in the dar
tutorial, if i understood it correctly, i'll have to write a script to do
decremental backups, which shouldn't be a problem.

the 7tb is compressed, and uncompressed, being text, is much, much larger,
and using btrfs transparent compression leaves a lot to be desired when it
comes to stability (i'm starting again after losing data after a very brief
power-loss).

i've tried using other backup tools such as rdiff-backup, however it seems
i would have to have all the files available uncompressed, as opposed to
adding each file one-by-one - does dar support adding one file to the
archive at a time, or did i misunderstand the tutorial?

some of the xml files are split into groups of pages, such as the english
wiki, because they would otherwise be very large files. sometimes a copy of
the recombined file is provided along with the split pages files, but for
the large english wiki meta-history file, for example, a recombined copy is
not provided, so writing a script to recombine it shouldn't be too
difficult (using cat and grep, etc.).

i want the decremental backup each month to only store the changes to the
large files, saving a large amount of space. does dar support only backing
up the difference of the file, as compared to simply storing a complete
copy of the same file when they're different?

aside from multiple backups, is there any data redundancy built into dar,
yet? (i'm worried about bitrot)