OK, I have now tested it.
1. On the first run, Caecilia would only display blank text for the Bold/Bold Italic variants at size 60. However, after another restart, all font sizes were displayed correctly. Helvetica displayed correctly for me since the beginning. Conclusion: this is not a bug in the font hack, but something weird happening on the Kindle. Restarting the framework might help for some obscure reason. Other than that, there is nothing I can do about it.
2. I'm still trying to understand your sentence "the Condensed selection would under some circumstances not shown." Is the button not shown, or does the selection have no effect? If it's the former, then please let me know. If it's the latter, this is a known problem (which I forgot to mention) in version 1.0. I now know how to fix it though, so this will be fixed in v1.1.