1. the size of any section of the graph doesn't necessarily dictate how many of that block exists (there could be any number of mob spawners)
2. blockEntities up to 3 chunks outside your render distance remain loaded and will still show up on the pie chart. This is why just dropping your render distance by 1 won't unload spawners/other blockEntities. You have to drop it by at least 4 to unload it. Example: If the spawner loads in at 15 chunks, you should decrease your render distance to 11 (press Shift-F3-F four times), then increase it to 14 (press F3-F three times). Reopen the pie chart and the spawner should be gone.

can you describe what exactly is wrong with gameRenderer? if i were to guess, you probably have entity culling on in minecraft video settings (it should be off)

julti is windows only

https://github.com/skillphobia/MacOS-multi-instancing-with-Keyboard-Maestro

tldr: yes

https://github.com/draconix6/Julti-CustomWall

this wouldn't be allowed unfortunately, however you may use a tool like BorderlessMinecraft: https://github.com/Mr-Technician/BorderlessMinecraft/releases