No. It's an issue of fairness, not a technical limitation.
I can manually make threads self-moderated (only I can do this), but I only do this if the thread is extremely long/entrenched and the OP is trustworthy, since you are not allowed to delete any posts made before the topic became self-moderated.
No. It wouldn't be fair or right to self-mod a thread after you've created it. Just lock the thread and create a new one if you wish to self mod.
I'm curious. we know that topic can't be converted to self-moderated or vice versa, but what will happen if you merge a new self-moderated thread with an old normal thread?
When merging, you merge from a topic into another topic. You don't add two topics and get a new topic. So it'll be whatever the destination topic is. When splitting, the new topic is not self-moderated. If you move a topic to a section where topics cannot be self-moderated, it loses its self-moderated status for as long as it is in such a section.