The sources are mostly from french here. I will obviously translate it as much as I can
What is a guru ?
According to linguists, the sanskrit term guru (serious) is a 'cognat'* from latin gravis that derivates from a indo-european radical meaning heavy. So the guru is a 'big man'.
The term gu means darkness and ru means lights that disperses, so guru means ...
I will give you some info on point 3, although its not a complete answer, consider it as a piece of information. You can find guru lakshan, shishya lakshan from variety of texts.
first the qualities of a shishya: From the Anadakalpa Tantra,
सच्छिष्यलक्षणं वक्ष्ये श्रूयतां ब्रह्मभेरव !। शान्तो विनीत:
शुद्धात्मा श्रद्धावात् धारणक्षम: ॥ समर्थ॑श्र कुलीनश्न ...