Installer silently ignores preseed directives

I'm trying to perform a fully automated installation of Ubuntu server on 100 Dell C8220s, by PXE booting the installer ISO then feeding it a preseed file over http using kernel cmdline params including (among others) url=http://myserver/path/ubuntu.preseed DEBCONF_DEBUG=5

I'm unable to defeat certain prompts; it appears as if the preseed values are simply being ignored. This is a showstopper to Ubuntu deployment at scale; we cannot tolerate any interactivity whatsoever with 100 servers. When (if) this works, it will expand to even more servers.

I've been banging my head against this for some time now, so I can supply a wide variety of preseed files that don't work, but which I think should work. I'll attach the more interesting ones to this report shortly.

1. Ubuntu release: Precise, using ubuntu-12.04.3-alternate-amd64.iso

2. Package version: whichever version of {debian,ubuntu}-installer or kickseed is included with the above ISO.

3. What I expect to happen:

3.1. Preseed directives shall either work, or fail in a debuggable way

Preseed configuration directives generated by debconf-get-selections --installer > file, when this file is edited and fed back into the Ubuntu installation, shall either:
3.1.a. Do what I want them to do; or
3.1.b. Output comprehensible diagnostics on tty4, explaining why they did not work.

For example, if I have the wrong owner name (not eg 'd-i'), the wrong directive name, the wrong type, or a unavailable value, tty4 shall show a diagnostic explaining which of these is the case.

3.2. The selections output by debconf-get-selections --installer shall reflect the choices I made in an interactive installation, to enable me to reproduce a previous manual installation.

4. What actually happens

4.1. In many (but not all) cases, preseed directives neither work, nor output any explanation as to why they did not work that is visible to me. Perhaps I'm looking in the wrong places - is tty4 not the right place?

4.2. debconf-get-selections --installer outputs great loads of stuff, which bears only a vague relation to choices I made during manual install.

For example, the output contains:
# Choices: Algeria, Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Cape Verde, Central African Republic, Chad, Congo, Congo\, The Democratic Republic of the, Djibouti, Egypt, Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Liberia, Libya, Malawi, Mali, Mauritania, Morocco, Mozambique, Namibia, Niger, Nigeria, Rwanda, Sao Tome and Principe, Senegal, Sierra Leone, Somalia, South Africa, Sudan, Swaziland, Tanzania\, United Republic of, Togo, Tunisia, Uganda, Western Sahara, Zambia, Zimbabwe
d-i localechooser/countrylist/Africa select

This is pointless; it won't defeat any interactive prompting, as it's missing a value. So there's no point including it in the output. It's just noise.
Also, I already told the installer I'm in Oceania, so I was never prompted for a country in Africa. This directive thus has nothing to do with what I choe in the interactive install.

On the other hand, the output also contains:
partman-lvm partman-lvm/vgcreate_parts multiselect

It appears to have forgotten that I selected /dev/sda1 as the sole LVM PV during the interactive install - the value is missing.
Again, without the value, this directive doesn't do anything useful.

I could accept this type of output if I had said 'debconf-get-selections --installer --all', ie asking for a sort of preseed template containing every possible directive. But I didn't (one can't).

I currently think that rather than using a preseed file, it would be easier to make my own initrd which executes cfdisk/pvcreate/vgcreate/lvcreate/mkfs/mkswap/mount/debootstrap the way I want. That's pretty sad - it's easier to write one's own installer than to figure out how to configure the existing installer.