I really liked Kelly during the pre season and I thought he did well when he came on towards the end of the season (think it was against SF when Mannion came on for Goff, which told everyone that we were going to run the ball, but still Kelly did well). However I am starting to have doubts about him. Every time there is any news on him, he is talking about skateboarding, seems more interested in that than football. Might just be the press looking for a story but if McVay has given him the OK to go boarding, then it suggests that he doesn't rate him. I wouldn't rule out Colburn, he looks dangerous in the highlights I have seen. Like you say, they might go with just 3 RBs in which case I think they would dump Kelly and put Colburn on the PS.
I also like some of the ILBs, Reader in particular but Patrick and Kupp are also interesting prospects and could see several of them making the PS.
I don't agree with the trade Higbee point. I really like Higbee and I think Goff likes him as well. I doubt whether he will get extended because of cap restrictions but he will still have a big part to play this coming season. I doubt you would get a 6th round pick for him and I think he is far too valuable to throw away for a 7th round pick.