Yes, each IDE would use its own default output folders within the VM.
Source code is relatively small, so size was not an issue. Besides, I typically used fixed-sized VMs anyway. But no, storing the sources on the host did not affect the VM size.
No.
For me, being able to compile the same sources in multiple IDEs was important. If I changed something, I didn't want to have to copy the changes to multiple VMs to test it, just change it in one place only and then recompile as needed.