I have asked them before regarding how long the process takes. They say they will be as quick as possible depending also if YOU have uploaded all the documents required and they don't need any further information.
here's the thing. All these little work arounds that you smart people keep coming up with, are the very reasons why they put these systems and restrictions in place.
When the day, not so far away comes, when you simply cannot get visas without going home, you'll wonder why 🤔
regarding airlines at Heathrow, they are pretty hot on onward flights within 30 days if your flying in and wanting a visa exempt entry. It's going to be their call if they will board you or not.
I believe they now send an email to say the application is successful or not and if they need more info within 48 hours before you send the application.
I would ring them and confirm how they now want it done before you post it.