Short answer yes. Longish answer. Yes, Because when buying on steam, You will still be asked to sign in with your uplay account. So don't worry about what site you are buying fromI assume buying from uplay cuts out the middle man and supports the company more? But that's your decision aha. Hope this answers your question
Yes you will need to add friends to your Uplay account all PC purchases require Uplay from any distributor.
Lots of PC gamers complain about Uplay but Ubi are stepping up their game now with enabling us to use Uplay points for discounts and of course the Uplay rewards for games i.e skins for certain games, weapons and wallpapers or sound tracks to games. Anyway I am going off subject.
Yes you will simply need to add steam friends to Uplay everyone will have a Uplay account for any Ubisoft game.
Because everything has been answered already, I'll just say that if you want to support the actual game more, buy it from Uplay or other digital distributors that have been approved by Ubi so I'd assume they get a cut as well, I personally use GamersGate.com as it has been suggested to me by one of the Ubi moderators.
Also, here's a list of trustworthy retailers that don't act as a grey market or key-resellers.
https://support.ubi.com/en-us/Faqs/0...il-Vendor-List