It looks like the locally mounted shares were created with 770 as you were concerned about in your last post but that shouldn't matter as apache and alex are both part of the users group. Did you see if you had a sabnzbd user and verify that it is also in the users group? If not that may be why it is not able to delete the files automatically.
As far as the ownership and permissions, I **think** that should be changed in smb.conf which is generated based on the hda_production database that you control through the hda dashboard. Check on your dashboard what the permissions are supposed to be for that share and change it if you can. If you modify the smb.conf manually it will be overwritten whenever the hda-ctl (I think it is called) service is restarted (manual, hda restart, etc.). But ... I am not sure changing ownership from apache to alex, deleting and then back to apache is necessary. I don't think you can change those locally mount shares through chown and if both accounts are in the users group, 770 permissions should suffice, I would think. My actual file folders and my samba shares are owned by my 1stadminuser (amahi) though so I can't verify. I just know I have had problems changing the local samba share permissions through command line.
If you have problems changing anything in the hda_production database I made a post here that details how to get into that table and make changes but as noted by bigfoot, it's not recommended by the Amahi team. I would try changing that line in the locally mounted shares service to 775 from 770 first (dir_mode=0770 to 0775 or hell even 777 as a test). It's a worth a shot before you go mucking with your database.
Edit: Out of curiosity, are using alex:users to try and delete the files manually (doing it outside the script)? What happens when you try to modify the contents in that share with a remote system actually connected using SMB (presumably then using account alex) over the network and not trying to access/modify the files through the locally mounted samba share in command line?
What happens if you try to delete the files in the share, from command line, when elevated to root/sudo?
Also what credentials/group are using to instantiate the locally_mounted_shares called from .smb_credentials?