I think the OP may be giving Grubbs too much credit for the resurgence of the OL. Recall, if you will, that McKinnie wasn't signed until August 24 of last year, 18 days & 1 preseason game before opening day to learn calls & settle in with the rest of the OL. Grubbs was (& is) a good LG but part of his value that year was as someone who knew the system & as McKinnie's bridge to the rest of the line, could help him acclimatize. Once Grubbs went out, McKinnie was playing next to (first) the disaster known as LeVoir and (later) Gurode, who were not only less able than Grubbs but were also newcomers trying to settle into the system.

In sum, I don't think retaining Grubbs would have been quite so dramatic an upgrade as the OP seems to think over either Williams or Harewood, & I expect (or hope) that gap will narrow as the season progresses & everyone gets more experience playing next to one another.

Leach, meanwhile, is a force of nature, but he can only block one man at a time, & when he's leading Rice he's also showing the defense where the attack is coming. Giving him carries & swing passes is a good idea just to give the opponents something else to defend against, but maybe the team should explore a bit of misdirection keyed to him (keeping him in to pass block, faking dives into the line while Rice bounces outside for a flip from Flacco, etc.).