So, I like that there's a KDE version and that is arch based, everything is just as I'd like except for the repository, it's not the same one as the official arch repository.

Does that mean it has every application arch has, except with more bug testing and adjusted to manjaro, or does it have its one selection of software?

Is it safe to assume that if there's a piece of software that is available on arch it's on manjaro, or is that not the case in the slightest?

I don't want to use the community repository by the way, I know you can use the AUR, I just want to stick to the official repository.

No, Manjaro does not use the same repositories as Arch.
I'm guessing all packages are based on Arch packages, and the idea is to have more bug testing, but I doubt they can keep up with it for each and every Arch package. So in a way it's a "selection".

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It's pretty easy to migrate from Manjaro to Arch or vice versa by changing the repos and a bit of pruning (there's a number of suggested ways out there easily searchable), however, you might experience some weird issues. The main differences seem to revolve around some of the Manjaro hardware utilities and the default use of grub, and updating via pamac. In other words, you can certainly make your initial install easier by installing Manjaro then switching the repos to Arch. Just don't expect any help with your problems from the Manjaro or the Arch people if you do!