A 12/3 homerun supplies circuits R21 and R22. These 2 circuits supply a storefront. R21 supplies 3 receptacles controlled by a switch. R22 also supplies 3 receptacles controlled by a switch. They only supply the receptacles shown. 600.5(A) states "Outlets must be supplied by a branch circuit rated 20 amps or more and that supplies no other load."

1. Is there any code issue with sharing a neutral to supply these 2 storefront circuits?

2. Are there any other flaws in this storefront plan?

Your reading the wrong section for show windows, look at 210.62 and 220.3(7).

Your seem to have enough circuits and quantity of outlets, but I don't know the length.

Multiwire circuits is fine.