The quests "The Banshee's Champion" (ID: 61486), "A Valiant Effort" (ID: 60003) and "A Message from the Justicar" (ID: 62162) show up on Horde characters as "Not Collected" with account mode enabled and account wide quest tracking disabled (see screenshots).

Well, yeah, because Account Mode shows you Things from All Characters.

Yeah that behavior makes sense for appearances, pets and achievements, for example, but it's a bit strange for quests IMO. These are quests I'll never be able to complete on this character (note that I've disabled account wide quest tracking in my settings). This means that if I enable account mode and disable account wide quest tracking, I'll never be able to reach 100% even in theory. Is this really intended?

It is intended. I mean, you can apply this to your other situations as well, like appearances, pets, achievements. If I tell ATT "show me anything any character can be used to obtain" (Account-Mode) and at the same time disable "consider a Thing complete if any character has done it" (Account-Wide) then I will definitely have many gaps in what I can actually complete on my current character.

So trying to account for every logical combination of settings in order to allow someone to 100% their given selection isn't really viable, and would definitely just introduce further lag from all the calculations needing to be done behind the scenes. Plus it would be even more confusing to purposely hide obtainable Things while in Account Mode.

If you are looking to sit down and complete a bunch of Things on a specific character, Account Mode is NOT going to help you in that regard, specifically because you will see Things that your current character cannot complete. This is why there are a variety of key bind options in place to allow quickly toggling various settings to meet the needs of what actions you are currently taking.

Thanks for the responses guys! I now see and understand your side in this. Thanks also for all of your efforts in creating this awesome addon!