1. ZiS-3 76mm: When firing barrage, if a shell hits a tank, is it the same damage has a normal shot ? I don't think it's the same, what's the difference ?
I don't think it is the same, i've seen shells hit tanks and very little damage. Don't have the stats for it though.
2. Crushing: I know the T-70 has pretty good crushing stat, which other tank can crush and which ones are the best, are pinned units more likely to get crushed?
All tanks in the game can crush I believe. Yes - pinned and HTD are particularly vulnerable as they don't move.
3. Mortar Barrage: Beside shooting faster, is it more accurate ? I can test this but after using barrage, is there a delay before the mortar starts firing shells again ?
No idea, it certainly doesn't feel like it. I have always assumed it's a rapid fire in a certain area (kind of like a rapid attack ground order)
EDIT: To support this see:
COH2 DPS Spreadsheet Credit to Endeav and wooof.
This shows AOE Distance & Accuracy as the same during regular fire and barrage, but a higher dps to the faster rate of fire.
4. Correct me if I'm wrong, but flame was very powerful vs. infantry with armor in vCOH, is this still the case ?
Flamers ignore Armour, so yes they are effective against PGrens and Shocks. (And there is the 10 - 15% chance of igniting a model instagibbing it)
5. I saw different information concerning "Oorah" in one post, I saw that it increase combat effectiveness, is this true or does it only increase the movement speed ?
It actually decreases combat effectiveness to a degree - when standing still it does nothing, however when moving it ofc increases movement speed, and during this the conscripts do not shoot.