Regarding the ".USER_DOES_NOT_WANT_MYNOKIA" file, can you make it ".user_does_not_want_mynokia" instead? The "shouting" of upper case is a little distracting when doing an ls -a.
Do you have any ideas as to whether it is possible to programmatically clear out offending pending SMSes from /var/spool/sms/outgoing and /var/spool/sms/regs_out.txt, for the situation where people boot with a flashed eMMC and no SIM, or even, if you can get in early enough, if they boot with a SIM? I guess you would need to be careful to only target My Nokia SMSes.