Length doesnt really matter. No bonus = no obligation. They aren't likely to resign Betts, and Portis has taken a ton of work these last two seasons, so he will see a ton of rest this offseason. Id be surprised if he made it to camp.
Dont know why I didnt figure this out til just now, but Lumsden, being in his 2nd year, is still an exclusive rights FA, and his rights would have belonged to the Skins after this year whether or not the contract was for 1 year or two. So, theres a better reason why the length of the contract doesnt matter, besides there being absolutely $0 guaranteed.
Lumsden did rush for about 5 yards a carry in the CFL and was the starting tailback by the end of the season for Hamilton. But the only CFL backs that have made the NFL in recent years were superstar backs who really were nothing more than 3rd down situational backs in the NFL(Minter and Fenerty).