Hi Dark Jobber, welcome to the Fang. Grab a tankard of Mjord and a slab of meat on your board. No fancy spices but that's the best way to eat your red meat really.
Right now, there is absolutely no concrete proof or announcement of the coming of the Space Wolves, just a lot of good conjecture and prediction based on past trends. So whatever you hear about Wolves is strictly speculation.
That being said, trends are noticeable and somewhat predictable, with GW becoming more "friendly" and relatively faithful to their word, so the best and most agreed rumors of Space Wolves is that they WILL come in 2018. Whether Russ comes or not, well, that's what we're all hoping as Guilliman remains the only loyalist primarch on the scene, and fluff wise, I hear he maybe having a nervous breakdown. LOL, Kidding, but fact is he is only ONE primarch holding the Imperium together. I would really like Russ to return to help him because ironically, apparently Guilliman has always liked and respected Russ whereas others cannot see beyong Russ' image of a savage.
The "dual form" of Russ originated far earlier than the release of Morathi actually. Opinions of what Russ will return us is actually quite polirizing among wolves fans. Some want him to come back as a "normal" Primarch similar to Papa smurf, whereas others feel in order to fight the daemon primarchs, Russ will need a "monstrous creature" form aka Wulfen.
With the introduction of Morathi, this has created a sort of hope for both camps, as it would satisfy those who want Russ to come back as Odin, as well as those who want Russ to come back as a giant furry.
for me, I just don't want him to come back JUST as a furry, preferably he has the ability to change form like Morathi.
In fluff, there is already one Wolf Lord, Brand Redmaw who is famous for being able to control his wulfen curse to the extent he has a werewolf form, and can revert back to Space Marine after battles, so the whole transforming thing is somewhat logical in fluff.
Well. The primaris seem to be recruited during the Great Crusade so they could be fenrisian. But I think first SW before finding Russ were from Terra and selected from different regions because they were searching brutal men.
I think original primaris are not made like the normarines, directly made from modified geneseed and organs. For example, BA's primaris are "standar" primaris with no blood rites in their process. Maybe SW primaris are free from the Canix Hellix (not sure if that was the name) process and effects.
EDIT: Sorry, posting about fluff in English is a real challenge.
Regarding the Primaris, the stories are still not very good on merging them with the chapters. For the Dark Angels there is no problem, only suspicion, and in any case, they segregated the primaris marines from knowledge of the Fallen. For the Blood Angels, it is implied that VERY recently, the flaw may have started manifesting in some if all of the Primaris Blood Angels. This is actually a small footnote in their timeline where Corbulo apparently after serving with the Primaris in a campaign, went back to Baal to report some "disturbing news".
Meanwhile, there is a chapter of Primaris Space Wolves called Wolfspear, who have been operating independently but longs to have better ties with their parent legion. Apparently most of the Primaris were "recruited" during heresy and post heresy era, so they ARE fenrisians. Not much backstory about them apart from being deployed FAR away from Fenris. The off comments seem to imply that Logan did accept some Primaris reinforcements into the Space Wolves. Whether they are accepted is not mentioned, but I've always believed the Fenryka to be pragmatic warriors and regardless of origins, will still respect martial strength and honor when they see it, even amongs mortals.