So my question: Why does Ubuntu 14.10 not allow to having installed both programms LibreOffice Writer and OpenOffice Writer?
That is not true. It is perfectly possible to install both. See Is it possible to install OpenOffice and LibreOffice in parallel?
Both might want to use the symlink /usr/bin/soffice as a shortcut (and you can only have 1 of those at the same time) but that is easily solved by manually changing the target.
The development in the past year has been very high on Libre Office. Open Office was not maintained for about 3 years, got hardly any updates (if any) and when Libre Office got forked (due to licensing concerns) a group of volunteers started adding new code like crazy (see for instance Why is LibreOffice default instead of OpenOffice.org for a list of concerns). They probably made a mistake somewhere where it concerns these 2 fonts/the 2 unicode ranges.
Or why is LibreOffice Writer not as unrestricted like OpenOffice Writer concerning fonts' display?
I do not understand this question. What is it you are asking?! I do not get what you mean with "unrestricted". To me this is a bug. Bugs happen. Well not in Open Office but that is due to the fact it does not get updates...
The link you point to concerns a possible bug filed today (it is not even yet accepted as a bug and is in the "need info" stage) and it is specifically targeted against 2 fonts ("Ahuramazdanew" and "AhuramazdaPlusTranslit") where 1 of them is not even released. Or in a broader view against unicodes ranges hex-10b00 to hex-10b3f
and from hex-10b78 to hex-10b7f (from the link). Seems a problem that has a rather low impact.
And seeing how quick a lot of things get picked up by the Libre Office maintainers I would expect this to be addressed quickly too.
