I did read the rest of your post but this bit didn't make any sense to me.
It's not a policy, it's a negotiation between McDonalds employees and the company. All Corbyn is doing is highlighting the demands and sending his support.
You might argue that it's not sensible to do so, but it doesn't demonstrate 'fag packet economics'. Wait until the Manifesto is released then decide if it is sensible or not, effectively costed or not, achievable or not.
The economic arguments are starting to look very smoke and mirrors throughout this current election - as they were in the last. Labours last manifesto was fully costed, much it seemed to the annoyance of many journalists who wanted to jab holes in it; instead they picked specific figures in it for 'gotcha' moments when MPs couldn't recall specific numbers they were being asked - and when they checked, the repeated line was 'why are you checking, don't you know'. Awful lazy journalism.
Meanwhile, the Tories, like the last election, are allowed to offer little detail of costs or numbers presumably as most journalists don't actually believe or care that they will do what they are saying. Much like the 40 hospitals claim that's repeated ad nauseum but has been inconsistently challenged.
On a final note, I find it laughable that those championing Brexit precisely because of the strength of the UK economy and the 'tremendous' growth we have, simultaneous believe that we can't afford policies designed to help everyone in the country. Or, that the amazing deals we have been promised will do post Brexit won't be able to pay for it all.