Evasion works with a "roll" system. Each attack generates a hitroll, which is then added up against your evasion. When the hitroll number finally goes over your evasion, you're struck by the attack, and the hitroll number resets. For some reason, they still express this as a percentage to dodge, when it's not probability based at all.
In addition, evasion will not help you at all once an attack penetrates. It is very much possible to be one-shot by bosses such as Brutus, ESPECIALLY on the higher difficulties. For these reasons, I find it very difficult to recommend pure evasion for a non-Ranger in solo play. In group play, you can have someone else taking the brunt of the damage.
I don't have much experience regarding energy shield, but I do know that it bleeds through on attacks that deplete it. That is to say, if you have 50 ES and get hit for 100, 50 damage will bleed through and strike your health directly. It seems to be nothing more than an addition to your existing health pool. Beyond that, I can't really comment.
Regarding armor: All forms of armor-granting gear have a strength requirement. It is possible to increase your strength on the passive skill tree, but given the Shadow's starting position on the tree, doing so would make your build less efficient on the higher difficulties. Armor offers a straight damage reduction, though, so if you're going to be struck by something, armor is the one value that will actually reduce the amount of incoming damage.
If you feel the need to go for armor, I would highly suggest ditching the Shadow, as its starting position on the skill tree makes it difficult to pursue STR boosts without damaging the character's long-term sustainability. Given that you like to play a ranged style, I would recommend rolling up a Duelist and working on a ranged build, as the Duelist's starting location is ideal for mixing DEX and STR (being a hybrid class of the two stats).
If you decide not to go down that route (understandable), I would just maximize evasion and not worry about Energy Shield at all. As such, Dexterity would be the best stat to focus on. If you've got a lot of blue skill gems you want to keep around, taking a couple of INT nodes on the skill tree would allow you to keep using them without overly damaging your long-term viability.
To answer your final question: Marauders can be DPS oriented or tank oriented. It all depends on how you want to play. Neither build is inherently advantageous.
%DtM happens first. Scenario 1 is mostly correct. To be technically correct, 100 damage gets you 10 mana, then deals 100 damage, 70 from life and 30 from mana.
The only reason this actually matters is for situations in which you are low on mana, or full. If you only have 20 mana in scenario one, you take 20 damage to mana, then 80 to life. By applying %DtM first, you gain that 10 extra mana so that it can be taken by MoM. However if you are full on mana, you can't gain the 10 bonus, so it is simply lost.
Scenario 2 is wrong because you don't take 70 damage with MoM, you take 100 damage. It just subtracts from two different pools.
As an aside, if you had 100 energy shield, that damage is absorbed by the shield and doesn't trigger MoM, so you would lose the 100 ES, and nothing from mana.
Best Answer
Armour is applied to physical damage before it reaches your Life or Energy Shield.
Here is a quote from one of the game developers, Mark_GGG:
Also FYI, armour does not reduce a certain percentage of all attacks. The amount it reduces is dependent upon the damage dealt. In order to get a 50% reduction of a 150 damaging attack you would need 1,800 Armour Rating. 1,800 Armour Rating would reduce a 300 damage attack by 33% and a 600 damage attack by 20%.