1. WeChat is heavily patrolled and monitored by the PRC censors. If the PRC doesn't want these rumors to circulate, these rumors would not last more than a hour (or seconds. It's very enlightening to see the sensitive terms get filtered in real time.)
In fact, in the last week or so, I've been getting more conspiracy forwards. The spread of rumors on this topic on WeChat is speeding up.
2. I don't see US' Secretary of State accusing China of intentionally creating the virus.