1. The highest GR mage botting can reach?
Mmmmmm. I guess it's 80, but please be aware that in GR 80 most bosses if not all are able to ONE-SHOT you, which means they hit me like 2.3M ONE-SHOT. It is not able to solved by combat routine nor gear. I don't think botting 80 could be an efficient way to run, but of course if you leave 300+ keystones I believe that you can pass 80 at long last.
2. Did I changed the Trinity or Combat routine
Nah, the default trinity 2.4.19 is nearly perfect and you don't have to do anything.
The setting are almost default as well I'll post some screenshots when I get home.
Oh, I also run with DB 445 (The newest release as the time I'm typing down this) as well
3. Why Unity?
As I've explained, DB can't make the best use of ice armor(if you have ever watched any manual playing mage videos on youtube you will find that human player can keep at least 10+ or 20+ trashes following them so they are always benefited from ice armor. I don't think db can do it for now.) so Unity is a MUCH MORE STABLE defensive choice. Trust me I've tested over 100+ GR 78s.
4. Why not Area damage?
Same reason, if you have ever watched manual playing mage videos via youtube you will understand why Area Damage is so important for them.
However it doesn't work for DB NOW(may be it works later). But actually my gear has like 40 Area damage

, you can stack it as you like, it doesn't make much difference.
Attack speed allows more survivalbility but it's not a must or something you should stack.