1. How strong is synergy between professions? Like class A that strongly relies (or is even unplayable w/o) on class B (warrior/healer in wow for example), or CC synergy (mage/rogue etc).
Synergy is very strong between professions, but not in a total dependent type of way. It isn't like the examples you posted, instead you can function on your own well, but you benefit greatly from different Professions being with you. You definitely want to bring a good mix of the Professions, because they all have something different and unique they can bring to the table, but there is no situation of I need this Profession to be paired with this other Prof. or else it will not work. It's a lot more loose then that, which is really good imo.
2. Do you think lack of diminishing returns on current abilities will promote stacking certain classes (5x necro traited for fears, or guardians).
Every form of hard CC in the game can be countered by stability, except for fear. However, a 5 Necro team would really lack a lot of diversity and would be punished for doing so. Also, it is important to note, that there aren't many moments that it will be 5v5, 4v4 is a lot more common then that, but the most common team fight is usually 2v2 or 3v3. So putting all your eggs into one basket being small duration fears could be very harmful for your team.
3. Are there any unique but irreplaceable abilities/skills for certain professions that will make this profession "must have" in the team
I will say this, each Profession is so unique in what it brings, that it almost feels like you don't ever want to stack Professions. Now, there are some good set ups with a few stacked Professions don't get me wrong, but that usually revolves around gimmicks. The only thing in the game that I would say is a must have right now is Venom Sharing Thief. That was the one broken thing I ran into that I believe was a must have for every team. Other then that, it was more about fulfilling roles well.
4. Looking at current abilities do you see any potential candidate for "its gonna be hard to balance", not sure how to describe it (smoke bomb equivalent)
Not really, so far it seems that everything we've ran into if broken can be fixed with mostly numerical tweaks which is very easy.
5. Do you think having "combos" that different professions can do together adds another layer of skill cap, or is it going to be something obvious and used on daily basics
Yes it does imo, obviously some combo field effects are stronger than others right now. However, when they all get balanced out appropriately it is definitely going to add another layer of depth that the top teams will surely utilize.
6. If it's not somethign confidential for your current team XD, whats the pattern for picking up professions to your team
It's all about fulfilling roles man, you look at a role you need to be filled in a certain map, and then you figure what you think would be best with the player assigned to that role for that map and for the match up you will have.