Lots of points to address. I will start with a few.
There is no need to build the whole bell from firebrick; most of it can be ordinary clay brick, with just the part above the riser top level firebrick. If you build a shorty core, it appears that you only need to have firebrick on the part of bell wall that the (horizontal) exhaust hits. The whole combustion core needs to be built of firebrick, with a possible exception of the riser which can be made from ceramic fiber blanket or Morgan superwool (safer to handle before firing).
The shorty core does not use a floor channel with its spalling risk, but it does require a specially welded door frame for controlled air supply.
The shorty core development is described in the
Development of a Compact Batchrocket Core thread. There are links to construction drawings scattered through the thread.
All-barrel systems do not have thermal mass. Single-skin bells have decent mass, but will take some time to start heating the room (maybe a half hour). I would advise a hybrid bell with either a barrel or a good-sized steel panel built into the bell, for some instant heat delivery. The more often there will be gaps between occupation days and the more often occupation will be short like just a few hours at a time, the more I would go toward metal in the bell.
"Steps" refers to jumps in system size; these systems can be built in any size from small to huge and still work, using the same proportions. A minor jump in size gives a significant increase in power. Going from 6" to 8" might double the power (just a guess). 8" is a very powerful core, and 10" would be a monster suitable for extreme circumstances.