You could try creating File Settings - Syntax Specific for .css files that, effectively, disables the default auto-completions. Copy all the auto-complete settings from File Settings - Default and tweak them until it behaves, relying on Tab for ZenCoding.
There are a number of Snippets (in folders) in Packages\CSS. If you don't use these then you could move them out of the Packages area. (They will possibly be reinstated when you next update ST.)
ST seems to have a number of CSS completions *hard-wired *into the app, but if you disable auto-completions then they should interfere less.
I can't help you with the folding issue. Andy.