Hello JohnKon and welcom to the ArtRage forums 
So to your questions;
1: No, not as such anymore.
AR used to ship with a full and comprehensive PDF, launched via the Help menu item, but that got cut back to being a Quick Start guide now which can still be launched from the Help menu.
Everything now is online, and yes there are lots of references still to be found regarding AR4, but a lot of what applied to AR4 still applies to 5 & 6.
The main thing about v4 that doesn’t apply now is Workbench Mode, that got replaced by the current Docking Mode when AR5 launched.
All Online Help files can be found here: https://www.artrage.com/manuals/gett...tting-started/
2: The Pallet Knife is the general Blending tool in AR, though it can also be used to lay colour down too.
As of AR5&6 the Custom Brush can also be used for blending. And in AR6, as an Eraser also.
Real colour blending is a global setting in AR and can be turned On/Off by going to; Tools > Colour Options > tick/untick Real Colour Blending.
3: Any tool or canvas can be saved as a preset and are stored in the appropriate folders within AR’s own folder structure.
These folders can be easily accessed from within AR by going to Tools > Browse Content Folders.
Different collections of tools, canvases, colour samples, stencils etc… can also be added to Toolboxes which in turn can be named and saved like a preset for use in future paintings. See Toolboxes: https://www.artrage.com/manuals/gett...and-workbench/
As for backups, well of course you will have your normal system backups you can always go to retrieve anything that gets lost should the need arise.
But specifically within AR’s Preferences you can set it to automatically save incremental backups of paintings each time you hit Save.
Because of how it currently works I would recommend keeping each painting you do in it’s own folder, as backup files are always stored in the same location as the painting generating them. More info can be found here under the section Store Backup Files: https://www.artrage.com/manuals/gett...4/preferences/
Things like custom keys, lists of tools linked to styluses etc… can also be exported and saved to where you want them.
Hope some of this helps.
Maker Of Replica Macoys
Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.14.6 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M