Yes, that's the reason why a dedup vdev that has lower redundancy than your main pool will fail with "mismatched replication level" unless you use the -f (force) flag.
I'm not sure about whether an L2ARC vdev can offload the DDT, but my guess is no given the built-in logic warning against mismatched replication levels.
Well, the warning makes sense with respect to the dedup vdev since the DDT would actually be stored there. On the other hand, the L2ARC would simply serve as a read cache, similar to the DDT residing in RAM.