If there's going to be a coalition to prop Labour up, it's going to involve the SNP. Even if the terms of going into coalition isn't another referendum on Scottish Independence (which it no doubt would be), they've consistently clocked up a deficit 7 times bigger than that of the UK and the biggest in the EU. The idea that they will be a moderating influence is fanciful.
As for the PLP wielding control, most policy doesn't go through Parliament these days and the PLP are terrified of deselection anyway so I'm not sure how much influence they'd wield.
I don't disagree that a strong Johnson Government would be a disaster but the saving grace (or not) is that after 40 years of Thatcher, Major, Blair, and Cameron, there is almost nothing to sell off anyway. So I'm not sold on the idea that Corbyn and the SNP would be any better: in fact, I think they'd be a lot worse.
Thankfully, you don't have to vote for either of them.