To answer your questions in order:

1) No, YOU do not need to level your Synergy; essentially, Synergy skill determines how much the impurities go up based on the difference between the item's level and your Synergy skill. The higher the amount, the more explosions you get, which is bad. As long as the person feeding fewell has a high Synergy skill, you're okay. The only things you're needed for are submitting the items - since augmented/ex items cannot be traded, you need to be the one submitting the items so you get the item back - and possibly your fishing skill, as synergy often has sub-crafts required. For the fisherman's smock/fishing stall, that's obviously fishing skill.

So to answer the second question; yes, as long as between all party members you have the required skills for the items (40+ Synergy, 60+ fishing for the smock / 80+ Synergy / 90+ Fishing for the stall), you can craft it successfully. HOWEVER, you do not necessarily have to hit these synergy tiers; you could, theoretically, craft it from Level 0 Synergy, but prepare for more failures than you can count. Generally it helps to be within no less than 5 below the Synergy requirement unless you plan on purifying impurities every other action. The other skill, I believe, has to be within the crafting minimum (I think you can craft something up to 10 levels below the required amount?) so that'd require at least 50 fishing for the smock, 80 for the stall. This is a guestimate though, I've always had the sub-crafts for the ones I've done. =x