Yes, let the tape go off first. Looked this up after the post from RandomGrinch.
This one, but I use 3 coats, not 2 as they say.
Get the ready mixed. Wet it down a touch for bedding the tape. Don't push too hard on the knife. It's all about getting the feel of the tools.
I normally bed the tape, then 3 coats on top. 3 coats on the screw holes, 2 coats on the junctions. Get the coats right and you only need to sand the...
It could be if he want to do what is shown in the photo. The flue needs to be at least 15% of the fireplace opening size. A common problem with doing what is shown is that not all the smoke he's up the flue.
Back in the 60's a lot of the houses were going up with Flettons. You won't find many in the reclaim yards, as they don't clean up too well.
They look a bit like the old Heathers, or possibly Chilterns. Hard to be sure from a photo.