Getting all email out from iPhone

Is it a wifi only iPad? If so, you can try to temporarily shutdown your wifi. If the iPad is not connected to Internet, maybe it won't try to fetch mails.

When you are prompted, have you tried to double click the home button and kill the Mail app? Or maybe try to access Settings from there.

You can also try to connect your iPad to your computer, if it's sync to iTunes. From there, in the Info view of the iPad on iTunes, select that you don't want any email account (I believe there is a overwrite current settings checkbox) and erase your accounts from there. You'll probably have to re-add your others account manually.

The attachments will not remain on the phone past the point that the email itself is in the default number downloaded by Mail (on my iPhone it's 50 most recent messages). Once the message falls out of that, and/or is deleted, its attachments should be removed from the phone memory.

The other way to be entirely sure they are gone is to disable 'Mail' in settings for the account you are using, wait a couple of seconds (let it do its thing) and re-enable. This will purge everything related to that mail account and let you redownload just the messages. If you download a lot of attachments versus the amount of mail you receive, it might be worth doing this on a weekly-or-so basis.