Mdadm looks for names of devices in /dev and always chooses the shortest name. So it will always prefer /dev/dm-XXX over /dev/mapper/XXX. While I appreciated that the latter way could be preferred, finding a generic way to prefer it would be a challenge.
There is no road map for "dm/md integration". Different people suggest it at different times, and they probably all mean something slightly different. Sometime people try to work towards it, but so far not with much progress. I'm not sure it would answer the above issue anyway. mdadm would equally prefer /dev/md0 of /dev/md/0, and there is no integration issue there.