We have to work with the arithmetic we're given, and yes, the "progressive alliance" doesn't have the numbers.
Do what they did in 2007 and announce that they're out, no deal, no coalition. I'll remind you what happened next. The largest party (the SNP) formed a minority government.
They can agree all they like, but if they don't have the votes to carry it, it isn't worth a button. They can go to the Queen and try to form a government on that basis if they like, but it's blindlingly obvious it'll have the lifespan of the proverbial snowball in hell.
The Tories will be trying to bring them down any which way they can. The arithmetic says they'll succeed, sooner rather than later. I'd be happy to be wrong about this, I don't fancy Cameron any more than you do, but a bit of realism is needed here.
Because the SNP and PC will not work with the Tories, and have said so. If the LibDems won't work wth the Tories, they should say so too.
Rolfe.