Hi April; Welcome to Permies!!
To start, Yes photos are always needed. Now, I'll try to give you some answers.
#1) How hot will the pipe from the transition area get? Way to hot to be exposed to any wood surfaces. At the least 200 F and at the most could be 600 F.
#2)Will a 6" pipe inside an 8" pipe filled with perlite be safe ? In my opinion No . You need a larger hole thru the wall, I would say 10" pipe with an 8" and 6" pipe thru would work with no insulation at all . That would emulate the old triple wall pipe at a cheap cost and " Should" be safe , careful checking with a digital temp gun would confirm that.
#3) If you can, use more than just cob to travel from the wall to the bell. surround your cob with bricks , rock . Rock holds more heat longer than cob!
#4) No leaks is no leaks however you look at it. Matt builds his bells airtight you
should as well. When we suggest how to design and build a
rmh First and foremost is SAFETY !
Carbon monoxide is a silent killer You MUST be sure you are not venting into your living area. All wood stoves have this danger it is not limited to
rmh's . The box stove you buy will have a disclaimer about this. As responsible rocket scientists we want to stress building these rmh safely.
We really need much better info and pictures to see just what your build is like and if it will be enough to safely heat your home.