There are a number of readers and Menu/launcher programs that run on the AuraHD, right along with the software Kobo provides. You mentioned both Coolreader and KOreader ,
(a guide to adding them and several utilities)
You can add them without harming your existing Kobo setup.
There is also an "Interactive Fiction" Frontz program "
QTFrotz" That is a reader of sorts.
Both Coolreader and KOreader have a longer list of file types that they can read, (I have a bunch of .pdb ebooks that both can read) KOreader reads PDF and cbz/cbr files well (I have used that feature to view .jpg images)
You lose nothing by comparing Kobo's PDF rendering with KOreader's.
You should check out the "Developer's Corner", as well.
Luck;
Ken