I have recently discovered that if I have CRW running on my sha port, it blocks UNO and MYR from generating blocks. As soon as I disabled CRW, they began creating blocks again...
That's bad news

Maybe that's the cause of no one mining Crown other than one/two pools.
Can the dev team confirm that merge mining Crown infact stops the mining of other merge mined coins?
After consulting with devs here is the answer to your questions:
getauxblock returns ChainID. In case of Crown it is "chainid" : 20
Any other MM coins that have a conflicting chainID can not be merge mined together.
The problem seems to be specific to these two mentioned examples, probably because of same chainid - would be awesome if you could check if it is also "20" - and thus is not a general MM problem caused by Crown code / mining.
the code relevant to auxpow mining is to be found here
https://github.com/Crowndev/crowncoin/blob/master/src/primitives/pureheader.h Here is an (old) list of chainIDs
https://bitcointalk.org/index.php?topic=769073.0 Crownfan