I did a restoration a couple year back. I use the old kamado 1-2 a year, around thanksgiving. this may help you out.
Q#1 - I don't have anything between the firebowl/firebox and the main kamado. I dont do any high heat cooks, all under 350F
Q#2. I've used the drilling a small hole method, so far so good. I've also used high heat epoxy to file the cracks.
Q#3 when running the kamado, i remove the plug completely. and use a modern top vent to control the temp. its worked well so far.
Hope this helps