Take a stab at it and let us know.
It doesn't work past the page you're on.
Yes it does; after selecting ALL look at the top of the page and you will see a message:
"All 100 conversations on this page are selected. Select all 329 conversations in Inbox", then click on it, and all your messages will be GONE.
If you want to reply, then
register here. Registration is free and your account is created instantly, so you can post right away.