Re: customer interaction
based on what you've told us, it sounds like your manager is being an arse.
First thing to do is prioritise your checking. Are you checking ALL scripts out in front of the patients or just the ones you call up to the counter? If it's the former then i suggest you to a slight re-jig of things and check your callbacks, deliverys etc at the end of the dispensing bench, thus giving waiting customers the hint that you're busy and less able to be interrupted.
Sit down with this arse and ask them what they think the best way to do things is. Then ask them if they think that interruptions when checking a prescription could lead to an error. Then point out what can happen to a pharmacist when the CPS wants to throw its weight around. Also point out that you seem to be working for them as the most highly paid counter assistant they've ever had.
Has this arse been involved directly with the events which took place or did they hear them all 'second-hand' from a 'helpful' member of staff? Does one of the other pharmacy team members have an axe to grind and want to set the manager against you?
Also, where the bloody hell is the counter assistant? Why on earth do they have a non-employee of the company standing at the front of shop wasting a large amount of time doing essentially what almost anyone else could do?
With regards to the written complaint - they happen. Customers frequently feel that you should be able to stop time, heal all diseases, circumnavigate the globe, cook dinner for them, deliver half the contents of the shop and know the exact stock levels of every chemist within a 70 mile radius, regardless of chain, all whilst doing their prescription, in your lunch break (ha!). Try to focus on all the people happy with your service and advice than the one moany git with nothing better to do than whinge on paper.
Remember this - you are a professional, as a locum you're your own boss, and the only time you need to start taking crap from people is either when you've genuinely done something wrong or when you can't easily find work elsewhere. Happily, this won't happen for quite some time. Cancel your booking and head to pastures new.
If it's a chain (the one with an R in it jumps to mind) then i would also flag this up to an area manager as a serious issue - after all, it helps give them a bit of ammo if they already know said branch manager is a bit of an arse.
Is that a record for the use of the word 'arse' in a post? 
i know i still hold the record for the most amount of times 'floppy cock' was used in a post 


“It's not worth doing something unless you were doing something that someone, somewhere, would much rather you weren't doing.”
Terry Pratchett