I know when you click the check mark and delete ur messages it only does the first page

i think it be usefull to have a delete all button of some sort

No from me. It is only cost 2 extra clicks and about 5 sec.

Not needed in my eyes.
In 2 times it's already fast, and at least you can have a second check of the pm who are in your Inbox. It can save you from deleting one private message that you want to keep.
Voted no.

what they should do with the pm area is like topics where you can choose how many posts (in this case pms) to view per page

that way you could set it to 50, and you wouldn't need this feature

i like your idea and voted yes but this may be a simpler alternative

this is a great idea aswell