1st : could we reach the same speed and accuracy level using one of those techniques ?? or should we use only the wrist downpicking to get a higher level ??

2nd : how to know if i'm using my wrist or my elbow to downpick ??
1 - Possibly, but the elbow will most likely feel much more tense and therefore it's a better idea to train your wrist.

2 - When you downpick, watch your arm carefully. Is your wrist rotating, or is the wrist joint straight and your whole lower arm (from hand to elbow) moving as one? If it's the latter, it's elbow. If the wrist is rotating, it's wrist.
It's preferable to only use the wrist for the picking motion, and use the elbow to move the wrist when changing strings. Picking from the elbow can lead to problems further down the line
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Picking from the elbow causes all sorts of problems + it's slower and less controlled than picking from the wrist. NEVER use your elbow to pick.
I started using my forearm/elbow when I was learning how to do chigging rhythms. Eventually as I got better I started using my wrist a lot more, it felt pretty natural. Wrist is usually best, especially if ypu are palm muting, cause you'll have to keep your palm against the bridge anyway. Just wanted to put my input :P
I kind of do both, and it works pretty well for me. Most of the work is done by my wrist, but I do move my arm into it to really dig into the strings and give me a thicker sound. It's like, 80/20 for me.

Totally using your arm can be pretty bad. Not only do you go slower, but you also get tired faster.
Here have a waffle!
If you watch flamenco players and then apply their technique to picking, it's all in rotating the wrist.