thing 1:
go into Settings > Mail > your gmail account > Advanced > Move Discarded Messages into and check Deleted Mailbox instead of Archive Mailbox
thing 2:
the Select All feature for deleting all your mail in a mailbox magically disappeared a little while ago. There is a workaround (a kluge really), but it means you really do have to delete ALL the messages in the mailbox:
go to the mailbox you want to delete all the mail from
select any one message
press and hold the move button, and while you are still holding it, deselect the email you selected
wait a few seconds
it will select ALL the emails with an option to move, and you can move them to the trash
Unlike the Select All method that used to be possible, you willn ot get a list of selected emails with the option to deselect some of them. So if there were a few you wanted to keep, you will have to go to the trash box, find them and rescue them.