“Axle cap”: what is it called and how to remove it

Regarding your question on tools to remove the track, this article (Natural Handyman - How To Repair and Adjust Pocket Doors) says:

Many pocket doors can be adjusted with special tools from around the top of the pocket frame with the door in a partially closed position. These tools are often specially angled wrenches designed to fit into that little gap. These tools are part of the assembly kit supplied with the pocket door, but are rarely given to homeowners by the builders. Without these proprietary tools, you have to improvise with or modify standard tools.

If you can't get to the warped part of the track with any tool that you have (or can find), then you will need to cut a hole in the wall for access.

This article (Ask the Builder - Fixing a Pocket Door) has a great description of how to get access to the track. The article talks about replacing the track but it also gives a warning that if the new track is not the exact same dimensions then the little project will become much bigger. So you will have to make a decision to cut the wall to get access to the screws to replace the track or just to get to the warped section of the track.

A quote from the article:

Remove the door from the pocket and take it off the track. Use a flashlight and peer up into the pocket cavity. There is a good chance you will see the screws that fasten the track to the frame. It is now time to cut an access hole on one side of the pocket wall. This hole will allow you to get your hand or arm into the pocket so that you can loosen and remove the screws and subsequently the track.

1. They're called hold-open washers.

2. It won't be easy to assume based on your description. Perhaps post a photo of where the washer normally meets the metal?

It sounds like the washer is probably just bent. Maybe you can take it off and straighten it in a vice and try it again. If it comes down to it, and you want to save some money.. you can back the set screw out of a drill stop and put a thumb screw in, grind the lugs off the shaft and slide it on!