What's the status of the TA partition? Was it backed up, and restored when you relocked the bootloader?
As far as I've read, restoring TA and relocking should be okay for warranty purposes. The only thing I've wondered about is you usually have to unlock via the Sony website, so they must know you requested the unlock key.
If the bootloader is locked with a restored TA partition, they cannot prove that you actually used it tho, so should be fine.
If you didn't backup, then restore the TA, then they may know.
As for the signal issue, had you tried another sim card to see if that worked?