About the DVD:
DVD+R is printed on the disk, as well as - separately (!) - RW
I burned the 14.04 LTS *.iso on to that DVD in Win7, following standard procedure as listed on ubuntu.com
Should this DVD still be re-usable to burn other things on it? If so, how much of the original 4.7GB space should still be available for re-use?
If what you mean for the "separately" "RW" is something like this:
That means, literally, "The read-write characteristics of this disk are: DVD+R". Don't let the logo (the stylized RW) confound you.
So in this case you have a write-once media; you can't erase data.
The writable-once disks come in two flavors, DVD+R and DVD-R. Only the first ones allows you to add data if the disk is not full (multi-session).
So if you have a DVD+R disk, the general answer is "no, you can't" if you have closed the disk, or "yes, you can" use the rest of the capacity if you have not closed it by opening another session.
In this case, the answer is probably no, you can't because I think that the programs that write a bootable DVD also close the disk.
A real rewritable disc will have:
