I can’t comment on the rest but Florian Cajori provides a view on use of ‘x’ in mathematics to denote an unknown variable. Cajori credits polymath philosopher René Descartes with the earliest published use of the notation in La Geometerie (1637). In the work, Descartes puts forward co-ordinate geometry (i.e. Cartesian geometry), which expresses graphs / shapes in algebraic equations. The work laid the foundation for further mathematical development and subsequent mathematicians maintained his convention of using ‘x’ (and actually other end-letters in the alphabet - y, z).

I will also say that there was a rival hypothesis that ‘x’ came from a Spanish transcription of the Arabic word ‘shei’ (meaning ‘something’ as referring to the unknown variable) - ’xei’ or simply, ‘x’. However, there are no earlier sources to support this hypothesis and it is likely popular fiction.

Source: Florian Cajori, ‘A History of Mathematics’, 1919