There is no way to 100% mute a string in a way that would allow it to be used again quickly and without the cellist stopping. In general all double stops need to be able to be played on adjacent strings, 3 string stops exist but are played as a flam from top to bottom as it's not easy to reach across 3 strings at once with the bow. The bridge is curved so to play 3 strings at once the middle string would have to have a ton of pressure on it.
Yes. If you are only playing the open strings you can depress the silent string towards the end of the fingerboard. Then you would need to have your bow placement higher than usual to avoid the depressed string. I wouldn't call this a recommended practice, but depending on your bridge curve it should be possible.
If you're playing pizzicato (meaning the strings are plucked with the fingers), yes! Unfortunately if you are playing arco (meaning with the bow), NO, however this may sound silly, but if you turn the bow upside down and play the bottom of the strings from underneath them, you can play on the C and A simultaneously without hitting the D or G strings.