for the first Q: If the md5sum for a downloaded iso is OK and the burned CD boots (not yet installed). Then is it safe to delete the downloaded iso ?
A: As i just learned the hard way as long as all checks out you should be fine to delete .iso images on HD.
Q: Also, can I copy the iso from the burned CD back to the hard drive should the need arise in the future to burn more CDs from this ?
A: from experiance yes you can either copy the CD from the CD or convert it back to an .iso image and reburn from HD. I use Nero to burn most everything and you can copy and burn any CD even if all you have is 1 CDrom "CD-RW". you can use a program to make an .iso image like winISO from a CD.