The Gr. συ, συμ, συν very often reflect Eng. co, com, con meaning same/sympathy μουσική σύνθεση > musical composition, coordinate>συντονίζω, cooperate>συνεργάζομαι , compete>συναγωνίζομαι, and many more.
Does the beginning συ means the same as in the prefix 'sympathy' meaning 'same'.
No. The prefix of sympathy=συμπάθεια (<συμπάσχω<συν+πάσχω) is συν where the -ν becomes -μ because of the π that follows.
Though that prefix sometimes also takes the form συ- (συζήτηση), it's not present in συχνό.
Not "same demand" but "together-request" I would say -- συν is "together" or "with" (like Latin "con-" from "cum").
No; as far as I know, συζήτηση is from συν- + ζητ- + -ηση and θέση is from θε- + -ση.
The -ηση and -ση are the same, though, I believe; an ending turning a verb into a noun describing that action (an action of requesting-together or an action of putting, respectively, from ζητώ "request" or θέτω "put").
θέτω was originally a lot more irregular, coming from Ancient Greek τίθημι which had a stem of θε- and no thematic vowel.
It is incorrect, yes.
"this common" is idiomatic English for "common to this degree".
Greek doesn't use αυτό and friends as adverbs meaning "to this degree".