We�re straying somewhat from the original question but I�m sure sandbach99 won�t mind as it is an interesting debate.
As I�m sure you know, magistrates have the services of a qualified legal advisor (usually known as the court�s �clerk�) to advise them on matters of law. Most matters that magistrates deal with are straightforward but where points of law are concerned it is the job of the advocate to provide the Bench with the salient points in order to make their argument, and for the LA to provide direction in the same way that a judge would advise a jury.
Yes, there are defences against the offence of excess alcohol, as there are for many offences. However, these are not so mysterious that a competent Bench of three magistrates could not grasp them.
The case you cite does not, to my mind, illustrate that the Sheriff was baffled by the complexities of the law. He had a simple matter of fact to determine and he decided that he did not believe the defendant�s contention that she had no intention of driving whilst she was intoxicated. Whether he decided this beyond reasonable doubt or (as he should have done) on the balance of probabilities, is not clear and that seems to be the basis of the appeal. I do not believe there was any misunderstanding of the law on the part of the Sheriff. It is hard to say with only outline details to go on, but I believe the same defence would have been unlikely to succeed in an English magistrates� court. It is clearly insufficient for the accused to simply say he had no intention of driving.
Those convicted by a jury are convicted by people �dragged off the street� (metaphorically, I hasten to add!). Juries have to give no explanation for their verdicts. Magistrates, whilst not usually legally qualified, at least have experience of judging similar matters and have to provide full reasons in open court for their findings.