Archimedes,
Tone is sometimes difficult to read in posts on this forum, believe me when I say I'm not attacking you personally, or trying to hack you off (although you might feel hacked off by what I'm about to say) but you come over as another person living in England who has no real idea of what's going on in Scotland other than the little reported by the English media. I know my (well read) friends and family in Cambridge and Surrey were totally surprised by what was going on vs what they read/knew.
You seem to find it inconceivable that the SNP could as much as triple their current number of 6 MPs. The latest poll in Scotland is here:
SNP support soars to record 47% | Herald Scotland
Neither is it a one off aberration. Current predictions say the SNP could well be on for achieving 40 odd MPs in the next Westminster parliament, and certainly puts them at 25+, most of the extra seats being taken from Labour. SNP voters are enthused, motivated and have a high turn out rate. By contrast Labour in Scotland is shell shocked and demoralised. A new leader will undoubtedly help, but Jim Murphy starts off 20% behind, and the SNP political machine runs very good campaigns.
Quite how losing 30+ seats in Scotland effects your polls predictions of a majority of 10 for Labour I don't know, but I would suggest a body of 40 ish MPs would make the SNP a major player in a hung parliament/coalition situation, ahead of the Liberals.
Throw in significant number of UKIP MPs, loss of numbers to both Labour and Tory, and the permutations for the next UK Westiminster parliament are many and varied. I wouldn't discount the SNP as a significant factor.