They could well be the largest party after the next GE, I just don't see how they get the required numbers in certain parts of England to win an overall majority with Corbyn and McDonnell in charge. Labour should be streets ahead in the polls atm, but they're not. All just IMO of course, but I'm pretty confident in that regard.
As for the bit in bold, Renton described earlier the 'managerial', risk averse strategy the SNP employ. I'm sure they'd want a watering down of his radical stance on Brexit. They would probably be broadly supportive of most of his programme, unless he goes full Marx on corporation tax and the higher tax rate.