1) No idea. :-)
2) Yes. Take your RH7.3 CDs one by one, and mount them. In RedHat/RPMS/, you'll find some RPMS called "qt-*", which are (probably) the qt3 ones, and some others called "qt2-*", which are the backwards compatibility ones for "old" QT. You may find you'll need to install more than just those. You'll probably need to copy them somewhere so you can install them all at once, with one RPM command line.
3) As above, but harder. :-)
4) There's more than one way to skin a cat. None of these are supported by RedHat. Or me. :-)
a) Using APT to upgrade RedHat:
Look at the "apt" package on http://freshrpms.net/
and consider - if you can - copying all the RPMs off all the CDs, making a repository, and doing apt-get dist-upgrade - there's a little more to do than *just* that, though. You'll need, as I understand things, twice as much diskspace as the RPMs take up, *plus* enough to install the packages. Good luck. :-)
APT is handy to have about anyway, incidentally. It's my RedHat Top Tip, even if it makes purist RedHat users a little confused, and Debian users *really* confused.
You can also use existing, online repositories, and judicous use of "apt-get upgrade" and "apt-get install", to save yourself disk space. I've upgraded boxes from RH7.1 to RH7.3 using this, and from RH7.3 to RawHide, too. It works, but you'll need to have some smarts about you, since the RPM port of APT doesn't *always* know what's going on.
b) Manual upgrade.
Alternatively, if you've got less space, try copying all the RPMS from all bar one of the CDs to somewhere, and do "rpm -Fvih */rpm /mnt/cdrom/RedHat/RPMS/*.rpm". Which *might* work.
What you're asking rpm to do here is Upgrade anything that's already got a previous version installed. But not install anything new. Since some packages will want entirely new rpms installed, this usually doesn't work straight off, but rpm will let you know what else needs to be installed, and you can fiddle, much like (2) and (3) above, but more so.
c) The Simple But Violent.
Finally, possibly the best way would be what we in the tech-trade refer to as a reinstall. Just backup what you need, and start over. No upgrade, just Install. Wipe the box.
With any of these, especially anything in (4), you'll want to have backups of your system. You may, with any of these, end up *having* to reinstall even if you didn't want to.