Well...this is what i think. I think Kahvi did enheirt some of willowgreens Magic. Maybe not the full blown healing buuuuuut...maybe just enough to force the conception when she wants. Cuz in SaBM it doesnt say it takes a year for her to concieve. I believe it says, Rayek waited a full turn of the seasons for Two-Edge to get better, then Kahvi announced she was prego and Rayek refused to leave cuz he "could have" been the sire (which we all know he was). So what if one night Kahvi and Rayek was getting it on and she was like "oh, dung he is leaving soon and i still havent gotten knocked up...i need this baby now!" and inadvertently released some of her unknown magic and concieved?
Also, As for a kid that would be a chief and unit both tribes? HELLO?? did we forget about Venka? After all she is Kahvi's Daughter, which makes her not only Two-Spears granddaughter but technically that is Cutters cousin. and technically she's the Blood of Six Chiefs..Whew...And not to mention she has Sun Villager Blood too...so she units the Sunfolk too!...well anyways...thats my take..