Instead of going to a category first, click on the waymark tab and then enter the location you want to search from. This can be an address, a city, or a zip code, or by clicking additional search options you can enter coordinates or select one of your saved searches. This will give you all waymarks in all categories.
As far as saved searches, this should be interpreted as being able to have multiple saved home locations. Over the years many geocachers have expressed a desire for this feature. I have a few because I travel for work and often to same locations, I can then just select them from my list.
The different search distances are set but the waymarks are sorted by distance from the center point thus if I only want to go out 20 miles I do a search out 50 miles and only go through the pages until I get to 20 miles.
To get a list of all categories click the category tab on top right of page and then click link View Entire List of Categories. See below.
As far as something blocking your edit page when entering a waymark, I am not sure what that would be as I do not have that problem unless I have the window too small then the items on right side of page are covering part of the edit box some. If this is the case just maximize the window and see if problem goes away. As far as hitting the back space key and ending up on a totally different page, we probably have all done that, just make sure that the text box you are in has focus or else the browser's default for a back space is same a clicking the back button on the browser.