There are now antidotes for Novichok and 3/4 of the Brit victims this year survived. There are nerve agents out there that have no antidotes. Doesn't mean they make a complete recovery either.
Havent seen any reports that Novichok induces a delayed reaction. Do you have any?
The Russian labs certainly would develop a version with a delayed onset so their intelligence agents can get away. It's what they've done for decades.
Meanwhile the Soviet Union broke up and the CW scientists sold their expertise and stocks of agent from the labs to make money _ novichok was available for a price and was sold to multiple buyers including Latvian mafia who used it to assassinate one of Putins business associates - per a scientist who had stocks of the stuff and sold it
The Russian inventor of novichok now lives in Princeton New Jersey and works for the US ( cw research lab)
Meanwhile various labs made their own copies of Novichok
Ukraine, Kazakhstan ( where the US seized their CW stocks), Czechoslovakia, Iran, and Porton Down among others