Why not use virtual machines? They are much less invasive, and you can easily remove them. The only downside is that it uses a bit more resources than you would normally, but it’s not a huge problem nowadays, even for budget computers as long as your host desktop is not resource-hoggy.
No. You are be able to do that, if you allow VM to access partitions as is, or whole disks. You then just, boot into them normally if you want to switch completely. There are also ways to boot directly into virtual disk images, and I wouldn’t be surprised if you could “convert” virtual hard disk image into a partition on your regular hard disk anyway.