@Evpok, regarding your question about the tau gallicum, I have no personal opinion of my own on the topic but I can't help noticing 2 things:
1/ The same letter "Ð" or "ð" came to be [used in Old English](http://en.wikipedia.org/wiki/Eth_%28letter%29) from the 9th century onward interchangeably with the thorn (þ). It was IIRC brought by Irish monks/scribes. The fact that they transcribed the Old English consonant with either þ or ð would suggest that the sound was /θ/ (voiceless) or /ð/ (voiced) as in Modern Greek and English or possibly /dʰ/ allowing for some evolution. Please note tha…