Oh dear you are getting yourself into a mess aren't you!
For onbar to work LTAPEDEV cannot be set to null!
And it will not work without the sm_versions file because then it has no way of knowing what version of what storage manager you are using. This use to be tables in sysutils you'd have to update every time but they made it a file to make it easier.
If you really want to backup to the blackhole /dev/null then you should put something like /dev/onbar and link /dev/onbar to /dev/null.