One other thing I noticed:
When going after family name, Kobo software uses "preferred [sub]family name" fields in ttf files - which can led to subtle confusion. For example "DejaVu Serif Condensed Bold" has:
family name: DejaVu Serif Condensed
subfamily name: Bold
but (the one chosen, subfamily presumably ignored and taken from the file suffix):
preferred family name: DejaVu Serif
preferred subfamily name: Condensed Bold
If you want to have that font even listed along the non-condensed one, you'll have to fix it yourself (ttfedit is nice simple app for such purpose).
EDIT (clarification):
All ttf/otf fonts have mentioned "family name" and if appropriate - "preferred family name". The latter is only present if it differs from aforementioned "family name", and is used mainly for supporting more than 4 basic subfamilies (styles). So the former is like standard, compatible naming that any app should be able to support - while the latter is more modern version for software / sytems that can use it properly.
For example - mentioned above DejaVu fonts can be listed as:
single family:
DejaVu Serif with 8 styles - where extra 4 styles have additional "Condensed" prefix - or
two families:
DejaVu Serif with standard 4 styles
DejaVu Serif Condensed with standard 4 styles
Now the problem I suspect with Kobo Touch - is that it goes after "preferred" family name (if present) - or the API it uses does so, no idea if it allows to choose - though expectedly it should if it's worth a broken dime
Yet Kobo Touch - by its design choice - is unable to support that, as the styles are not only limited to standard four (regular, italic, bold, bold italic), but they are taken directly from filename's suffix instead of the font's metadata. So in context of the above example - DejaVu Serif Condensed will never be listed along DeajVu Serif without altering the metadata to not confuse Kobo.