Metadata and custom columns

I haven't had much luck finding a direct answer to this through documentation or searches so I'm just gonna ask straight out and hope someone more knowledgable can point me in teh right direction.

I cobbled together a plugin hacked heavily off of the GoodReads plugin. It works pretty well for what I would like to do, but there is metadata I want to collect that isn't present by default.

So here are my questions:

1) If the plugin passes back metadata that the install does not have columns for, what happens? Is the extra data stored or dropped, or worse, an exception thrown?

2) Can the installation of the plugin create/validate the custom columns ?

and semi-unrelated

3) Can I send any kind of textual updates while the plugin is running to display with the "progress bar" ?

Document references or examples of plugins doing this would be fantastic.

Thanks!