Hold on Tiger! You're far behind the times, but I'll get you up to speed
First off, 3C>IAD Allecan doesn't work in CSE anymore because of hitstun deterioration on 3C.
2D loop still works, but it will waste a lot of meter because of the new change (brought into CS2) where using the same "ADA move" in a row uses more meter than using different "ADA moves". Therefore I would drop this since ADA meter conservation would be very difficult for you
2C>8D loop is still there but I'll show you a better 8D loop to use
What the hell was the 4D loop?
Volante loops died after CS1. You can only do 2 in CSE without the opponent teching out of the combo, and 2 volante's isn't a real loop
What's the difference between 2D, 8D, and Volante? What's the easiest Variant?
2D is the strongest starter of the 3
8D is the best finisher of the 3
Volante is used for moving in on the opponent. Combo wise it is usually used as a relaunch tool
As far as chosing which ones to use, you want to use all of them in a combo. Start with 2D, then use volante to relaunch, then use 8D as a finisher
As for the easiest variant, 2D wins since it gives you the most damage output in your combos. If you want an explanation as to why, read this nice passage by a guy named dragontamer#s
Basic Mid-Screen Combo+Corner
Okay, so in CSE you have several reset options that are simple. Here is one extremely simple combo
j.c= Jump Cancel
JC= Jump+C Button
5B>6B>Cantabile>5C>j.c>JB>j.c>JB>JC
Basic corner combo
B>6B>Cantabile>5C>j.c>JB>j.c>JB>JC>Brio>JB>j.c>JB>JC
Since you said no Allecan stuff, I avoided that
try these 2 smoke combos and experiment with ADA's other moves. The other combos are "advanced"
And for your final question, All of the CS2 Combos work, except for the 3D Loop since you can no longer repeatedly do JB>J.2C