With tx_news.templateLayouts
you override the flexform for the news plugin you insert on a page, not for a news record.
Your second code looks almost right, but you forgot settings
: TCEFORM.tt_content.pi_flexform.tx_news.sDEF.settings.categoryConjunction = and
And even though this code is right, it will not work because of a bug with flexform fields that have a dot in their name (this is just stupid). I think the only way to cleanly modify the flexform, is to write a small extension which overrides the default news flexform.