How to get list of all users in a site collection

In your url just append _layouts/people.aspx?MembershipGroupId=0. This will give you all users from all groups in a site collection.

If Url is http://localhost:9000/default.aspx, try with 
 http://localhost:9000/_layouts/people.aspx?MembershipGroupId=0

Now,If you want to delete a user from site collection, there is remove option in the breadcrumb menu

How to get list of names of webparts on a page

In your url just append contents=1. This will give you all the web parts that are deployed.

For example: if Url is http://localhost:9000/default.aspx, try with http://localhost:9000?contents=1

Now, If any web part makes errors on your page, you can disable/remove from this page