As with any election campaign a lot of it comes down to timing. Some of the timing is in the control of each party but most of it isn't. This is one thing the SNP should be in control of. We don't even know even the election will be yet. Although I think the main parties are now narrowing it down to Sept, Oct or Nov.
I'm not sure your point holds, as Labour not getting a full majority is the only way the SNP can have any influence at Westminster. I might have misunderstood what you are saying though?