I think Banks' defense put too much stress on LBs. It was rare to get a guy like Keenan Pili who was both smart enough and athletic enough to work on a Banks defense. Knowles "unstressing" the LBs will have the effect of 1) freeing them up to utilize their athleticism more and 2) allowing more athletic LBs to play more, instead of having to rely on an unathletic guy out there because he's "cerebral." I'm specifically thinking about Telander here...although I think that when Telander plays, he will look better because he won't be asked to do things that aren't his skill set. I'd look for the stating LBs next year to be Spillman and Perlotte.
As for point #1, we will have 2 good CBs in Redmond and Gibson. But (assuming Hood goes pro) you're right, we will need a 3rd from the portal, unless Merritt or Poteat develop. And I'm only talking about the outside CBs here. The Nickel is considered a Safety in this defense...we'll need to get a Nickel from the portal too...and 1 or 2 Safeties.