I haven't got my head around why Salmond's case decides whether Scotland has another referendum?
Not so much 'can as will. Firstly independence is still closely associated with Salmond. If he were to be found guilty, it's going to have an effect on the support for it, even if that isn't for rational reasons. but then rationality isn't that big in voting.
Secondly there is a split in the SNP about how to approach the next referendum. The part that is most closely associated with Salmond is also that which is pushing hardest for a referendum. A guilty verdict on any of the charges will push more power to the gradualists.
Thirdly, there is a belief that the process of the initial investigation by the Scottish govt, already demonstrably flawed as shown by having to pay Salmond's cost when he sued them, may also in terms of what is revealed by the court as to who knew what when, show that Sturgeon behaved incorrectly - this may well lead to her having to step down. And that's even if Salmond is found guilty. This is turn will lead to a battle in the SNP between the already mentioned factions which unlike the last few leadership elections is likely to be very damaging.
In order to get a referendum, I think the SNP and the independence movement as a whole will have to be united in thinking that a win is close. Most of the results of the trial will damage the reputation in some way, and it will be a media circus in which even the best outcomes will have distracted the party and the people.