The difference between 8221A and 8221B (hence 8221AR or ARZ vs BR) is shown in the electrical specifications tables and mentioned in the feature list on the first page of the data sheet. The B version is guaranteed to a higher performance level on DC parameters like power supply rejection ratio, DC offset and so on. Often we find it necessary to study a data sheet very carefully indeed to find the meanings of some prefixes and suffixes, but ultimately, they are there somewhere on the data sheet.
Typically ROHS compliance costs the manufacturer only a one-time charge to pass through the compliance process and possibly adjust their process or materials used. I'm not sure why this cost isn't passed on to the customer typically, but I believe that the costs are typically not very high, and that customers are not willing to pay extra for this.
REEL and REEL7 refer to the size of component reel the part gets shipped in. When you buy ICs for use in a factory, you buy them on circular reels (like the reels we used to see on reel-to-reel tape recorders) much like a film reel. These are standardized to fit into the various pick-and-place machines that assemble the parts onto the pcb. I don't happen to have links to the reference standards. A quick look at Digikey catalog shows that REEL includes 2500 pieces, while REEL7 includes 1000 pieces, so I believe that REEL7 is probably a 7 inch diameter, which is smaller than the normal ones, called REEL.