1. There are different approaches, it depends on the situation. I think what is usually the best is to solve all corners and as many edges as possible, and finish with edge insertion. But this is not always easy.

Another common option is to do "blockbuilding" and try to finish directly or leave a few pieces unsolve, like in a normal solve.

I have tried your first scramble:

R' U' F D' R2 F2 D' B2 D2 U B2 F2 U' L' D F D L2 B2 U R B2 F2 U' R' U' F

（F L' D' R2 B' R）//EO（6/6）

D2 F U F //DR（4/10)

R2 U' F2 //One square and many pairs (3/13)

R2 [2] //2 other squares (1/14)

Up to here it was promising, but then I had to break some blocks to go on:

F2 L2 D' L2 [1] U B2 U2 //2e4e (7/21)

The skeleton is not incredibly good. But, as often happens with DR, the insertions are very nice:

[1] = L2 D2 L2 U2 R2 U2 (6-4/23)

[2] = R2 B2 U B2 R2 F2 D F2 (8-4/27)

Maybe later I will try the other two. If I do I will edit this post.

2. I assume you are referring to what they also call "slicey shenanigans" or "free slices". The way I see it is this: a move like M2 solves a 2e2e (double swap of edges) case (ignoring centers). Just like you solve 5c (5 corners) using 2 commutators, you can solve different cycles of edges (for example 2e4e etc) using this special "2e2e" cycles. When I insert a slice move like this, I try to always solve at least one more edge. If you do more than one insertion, the first ones tend to be easy and you can even cancel moves (you have more freedom), but for the last ones sometimes you are not lucky and you need setup moves. This is a solve I did last night:

R' U' F D' F2 L2 R2 U' R2 D2 L2 U2 F' R' U B' L' D' F' U' L2 U2 F R' U' F

B' R' L2 F U //EO (5/5)

(F2 L F2 L B' L2 B) //DR (7/12)

(R F2 R F2 R' B2) //7e (6/18)

Skeleton: B' R' L2 F U B2 R F2 [2] R' F2 R' B' L2 B [3] L' F2 L' [1] F2 [4]

[1] = S2 (2-2)

[2] = S2 (2-2)

[3] = S2 (2-1) (2e2e4x in 19)

[4] = [L2 U2 R: S2] (8)

Solution: B' R' L2 F U B2 R B2 L' F2 L' B' R2 B' F2 L' F2 L' B2 R2 D2 L' F2 B2 R U2 L2 (27)

3. Some of the are easy to understand (for example commutators), but some others are hard to understand. There are many 2e2e cycles that I don't understand and I have just memorized (for example F2 L2 U R2 B2 L2 D R2), but sometimes I forget them

I hope someone else has a better answer for this.