Standalone Snowflake proxy and the browser extension Snowflake proxy are roughly equivalent in this regard.
Pretty low. Should be a bit lower than running a non-exit Tor relay, as I said in Snowflake abuse - #3 by WofWca. As said on https://snowflake.torproject.org/, your IP is not exposed, i.e. people can’t post naughty content from your machine.
Also see Do Snowflake Proxies Reveal Tor Traffic to ISP? - #3 by ukmr.
In this regard the risk is roughly the same as joining a call in a web app, i.e. pretty low. It all comes down to WebRTC, and WebRTC is made to be secure.
There is also the code that is responsible for connecting to the Tor relays, but the proxy will only connect to destinations that are controlled by the owners of torproject.org.
So, in this regard the risk is also low.