I'll make it simple. If I have 54.67% absorption, am I absorbing 54.67% of the damage? And if I have 52.01% shield chance. Am I going to shield 52.01% of the time when attacked?
Yes. 54.67% of the hit is absorbed if you shield. 52.01% of attacks that don't get defended will get shielded.
And what is the ideal stat scenario? 20% D, 50% Sh, 50% Ab?
Different for each class. That looks roughly like a vanguard's distribution. There are different approaches but my vanguard would look more like 15/60/60 with all buffs.