I'm happy to help ya.
Understand that the joists primarily span to hold the lath & plaster, and in your case, give some structure for flooring.
They also hold tension, w/o which the roof would push the walls out..
Again,
stiffness is the big concern and what is needed so that the plaster ceiling doesn't crack. -that would be a sign that there is not enough structure to provide adequate
stiffness for the weight. IOW, the wood will flex quite a lot before breaking, whereas the plaster is much less elastic. So the wood portions need to be over-built to be extra stiff. And that is what you have... as long as you don't start storing bags of concrete up there.
Think of the long boards as really there only to provide a surface to walk on.
Locking the joists together isn't a need. The lap, or tongue & groove, helps transfer a small amount of load to the adjacent board, when walking around. It also does away with straight cracks between boards.