"1. If, out of a total of 6 GB of RAM, I designate--say--3 GB of RAM for the guest OS, can I still use the full 6 GB of RAM for the host OS (PCLOS) if I have not yet started up VirtualBox?"
i think that while the machine is running the ram is used by the virtual machine but not before or after
if this didn't happened, then how could you run multiple virtual machines if you already gave all your ram to one vm and the second needs more ram?
"2. When a guest is created with VB, is that reallocated portion of the HD erased and reformatted (as specified by the user) for the new VB partition(s)? Or does the newly created VB guest portion of the HD still retain its portion of the original file system installed on the entire HD (e.g., ext4 for a Linux OS)--at least until the user decides whether to reformat and install something over it, such as an OS?"
remember that you are using a virtual machine, it doesn't have full access to the the system, only to ram and cpu, the rest is virtualized, including hard disk, the vm doesn't have any access to the real hard disk, the entire vm access only a file, this file contains the virtual hard disk containing the os, each time that you create a new vm, you can or reuse a previously created hard disk for the vm or create a new one
"3. To use the guest "portion" of VB, do I always have to install an operating system in it? Or, can the guest portion be used without an OS being installed there--for example, just for storing files or data?"
remember that the virtual machine is just a emulated pc without a os, if you don't have a os, how could you copy and paste files inside it? when the vm starts it is just like a pc without os or hard disk, it will tell you that it didn't found a os to do anything
the whole idea is put a os inside this virtual machine and do things that you can't or don't want to do in linux
"4. If I decided to delete the VB guest OS, could I also delete the associated VB partition(s)? If so, could those "deleted" partition(s) be integrated back into the Linux partition(s) on the host side where it/they originally came from? (By using, say, gparted?) Or, would that/those partition(s) be available for use only by VB?"
there is no partition, it is just a file store inside your home folder, you can keep it or delete it
remember that if you created a hard disk for a virtual machine of 100gbs, fixed size, then you have a 100gb file in your home folder, something to consider if you make multiple machines