Indeed. It's really a shame that this is not possible with Dell's official installation media. I can imagine that it might complicate support requests occasionally, but it would also be a huge selling point for many security-minded customers.
I need to decide what to do to to resolve this on my system, since I definitely don't plan to travel again without FDE. I don't think that home directory encryption is an acceptable substitute really, so I'm going to strike that option from my list. I'm a long-time Linux user and fairly technical, and I'd be happy to write up a tutorial for others (like the steps that you've worked through so far), and if it turns out that the only reliable solution is to do a fresh installation, I'd probably switch to Mint and write up the steps for doing that instead.
I hope that I can get FDE working with the official Dell Gnome installation. A lot of effort seems to have been put into getting everything working smoothly and I like the idea of participating in Sputnik. So I'll probably try to back up everything, reformat with FDE, and then restore it all first and see how that goes (and if it can be done that way, then I'll post steps for that).
One way or another, we'll get over this bump!