1. No - 10GBps should suffice fine for the vast majority of OSA workloads, the supported network minimums for OSA remain unchanged.
In vSAN OSA, typically only larger, busier and or/using things like R5/R6 (or R5+R5/R6+R6 in stretched) clusters would be exhausting the resources of 10Gbps networks.
2. This is the supported network minimum - I don't think there is any hard-check that prevents using lower than this but if you choose to run on an unsupported configuration then you are taking responsibility for supporting this yourself when/if things go wrong.