Scenario 1: Labour 330 MPs
Scenario 2: Labour 280 MPs, SNP 50 MPs
Scenario 3: Labour 280 MPs, LD 50 MPs
Scenario 4: Labour 230 MPs, SNP 50 MPs, LD 50 MPs
(You can adjust the numbers above based on your optimism.)
A) The numbers are there overall, in one guise or another
B) Neither the SNP nor the LD prop up the Tories instead (I really don't see why they would. A second coalition would finish the LD - they learned that lesson. And the SNP have zero reason to.)