How a SFC got screwed by the BN. Thoughts?

Well. First off. The CSM should only "advise" on the eval. In gross incidents of "non quantification" sure. But it is ultimately up to that rater. Same with awards. If higher makes a decision to downgrade, got it. But allow you Soldiers to exercise their decisions with guidance not directives.

Hope this SFC has IG on speed dial. Sounds like they have a good case for a complaint to be filed

No one has any authority to “make” a rater change a rating; that is completely on the rater. Sure, higher HQs can suggest adjustments and changes, but at the end of the day this is on the rater for the rater portion and the senior rater for the senior rater portion. A platoon sergeant’s rater is usually the PL, and the senior rater is usually the CO, so BN has very little to say about ratings. Once the NCOERs go on to EES, this sort of thing will cease to be a problem, as EES will only allow raters and senior raters access to an evaluation, and folks like 1SGs and CSMs and others outside the rating chain, who usually tend to exert undue influence like what you describe, simply won’t have the ability to be involved.
No one has any authority to “make” someone change an award recommendation to a lower award; that’s what the approve/disapprove/upgrade/downgrade boxes are for on the award recommendation form.
As many others have pointed out, there’s usually more to the story. Based on what you’ve presented, quite frankly, this sounds like a case of the officers in the SFC's chain-of-command (ie, PL, CO) not having much backbone or understanding of the authority they’ve been granted by the Army, and the CSM and BN CDR believing they have more authority than the Army actually grants them.