There are multiple route to get from point A to B. For example, from Versailles to Invalides, you can take: RER C then Metro 8. Or TER N then i can choose from two different metro line at Montparnasse Or TER U then RER A.

There is also buses, bus since the rail is faster, I never take it.

In case of strike the network is never fully down, people that can remote work do it, so there is a lot less of people transiting. On the biggest strike you can loose an hour or two while commuting, for small strike, it will get more crowded.