I can see Susan doing a good job of moulding her children without anybody registering it!

Yes, Mendanbar is much more easygoing than Cimorene, so he should be good for Daystar. But I think it's too bad that Wrede had boxed herself into a corner when she went to write the prequels to Talking to Dragons, because I think if she had written the series in internal chronological order, she would have solved the problem of Mendanbar's enchantment very differently.