Am intrigued by the PM’s letter. The Conservatives were absolutely clear in their campaign literature what a vote for the SNP would mean. They ended up winning 48/59 seats. What’s changed since December? 
Clearly the SNP aren’t in a position to affect a minority govt which is at least part of what the Tories meant. However there is a way in which their message can be interepreted (literally) and that might well cause them problems.
The basic strategic problem for the Scottish Tories is if you keep framing every election as “a vote for the SNP is a vote for independence” and the SNP keeps winning elections, then, well quite obviously you have an issue.
