Is it possible do enable/disable CSS completely in DW CC 18 with a simple click as in Safari (developer menu)?
No, not in DW, try Wappler, that can do it if I think what you mean is you would like a little checkbox beside every css style, so you can switch styles on and off
Yes in Design View only. Go to View > Style Rendering > untick display styles.
This has no effect on Live, Real-Time Preview or Preview in browsers.
If you want to completely disable CSS, comment it out from your <head> tags.
<!--<link rel="stylesheet" href="styles/main.css" type="text/css">-->