The difference between this case and those cases is that this is a free host. As in, it isn't my job to import your databases for you.
Are you saying that you have single INSERT statements larger than 1 MiB? If so (and note that 1 MiB of text is a HUGE amount), then a new topic created in your forum should give you that error. If not, then you should be able to send that same INSERT statement from PHPMyAdmin. Also, my guess is that PHPMyAdmin does not individually parse each statement in a query dump as a separate query to the database. It could easily send more than one statement in a single query.
As for the domain change: okay, it has been changed. In the future, pay attention when you are told (in red text) that "you only have one chance to create your account".