Yes, I agree with you she should have moved and it was not unreasonable of you to say "excuse me?" and expect that. But no one had "taught" her to move out of other people's way (like your mother and my stepmother taught us) and doesn't sound like she had a lot of common sense to pick up on how her behavior was impacting anyone else's or, she just didn't care.

However, theoretically it was still her turn since her groceries were still being "processed" so, that there was a problem I would blame on the store. Not having enough clerks or clerks working at differing speeds and providing a "comfortable" checkout situation is their problem and under their control. The customer is never wrong. You're the right size it's the pants that don't fit.

The bottom line is you came to the store to buy groceries, not to be forced to interact with other customers well, ill or in any other manner.